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The simple 15′ vertical antenna shown mounted on the railing of our second floor deck has produced almost 200 countries worked around the world… VQ9′s in Chagos and 3B8′s on Mauritius in the Indian Ocean, TX0DX on Chesterfield Reef, VK0MM on Macquarie Island in the Antarctic region, BQ9P on Pratas Island off Taiwan, ZM7ZB on Chatham Island in the South Pacific along with FO0AAA on Clipperton, 9M0OO on Spratly Island in the South China Sea, JT1CO in Mongolia and on and on. What I hear, I can usually work with this little wonder and the small size and profile make it feasible for use in deed restricted neighborhoods.

A radio amateur friend and antenna designer came up with a simple design for a 10 meter vertical, which another friend and I modified to make work for the 14, 18, 21, 24 and 28 MHz ham bands. Its performance surpised us, and I’ll share it with you, in case you too are looking for a simple, inexpensive DX antenna that really performs well.

Main Antenna Concept 

The basic concept is to put up a piece of aluminum tubing with a telescopic section held by a small hose clamp to adjust the height. By attaching the center conductor of a coax feedline to the tubing, and the shield of the coax to a couple of radials from the base of the tubing you can load the vertical across quite a broad range of frequencies.

Of course, with a vertical element of approximately 15′ this is a non-resonant antenna for the 10, 12, 15, 17 and 20 meter bands. I chose this length on purpose to allow the system to be tuned to resonance with an antenna runer.

Tuning

Since the SWR in an antenna system of this type will be relatively high, an antenna tuner unit will definitely be required. You may need an external ATU if the one in your transceiver can’t handle the impedance mismatches involved. Here at K7ZB, I drive my TS570 (which has a built-in ATU) thru the amplifier, which then drives a high power ATU to the antenna. I put the SWR/Power meter between the amplifier and ATU to ensure a good match for the amp, and in cases where I run barefoot without the amp, I can still use the ATU to assist the transceiver’s ATU in ensuring a good match.

In this way, everything is matched for maximum power output: from the transceiver to the amp, and amp to the antenna. And, even though the SWR’s are high at the feedline and the antenna, it doesn’t matter because the system is matched with the ATU.

Mounting Scheme 
The picture below shows the details of the mounting scheme.

This picture shows the center conductor of the vertical connected to an SO-239 female coax connector. I used two pieces of insulated #14 AWG solid copper wire to provide a stiff means of supporting the connector to the metal bracket. Note that there is no true ‘ground’ connection to this antenna. The ground side of the connector simply connects to the hardware bracket, to which the two radials are connected. The bracket looks like a simple piece of offset metal used to mount a small flag pole or the like.

The two ~15′ radial wires are held to the bracket with a large sheet-metal screw, so the bracket is connected to the coax shield. Electrical isolation from the center conductor of the coax connected to the vertical element is provided by an insulating rubber sleeve. This is a piece of neoprene fuel line chosen because the dimensions fit the aluminum rod inserted into the lower 14″ of the aluminum tubing. However, we found the electrical isolation properties of neoprene fuel line leave a little to be desired at the high SWR’s of this system. After driving this vertical with 500 watts at high SWR in the middle of one of the DX Contests, I punched through the insulation. Obviously the original 10m antenna design was intended for lower power and lower SWR’s! This problem was solved by wrapping the neoprene sleeve with several layers of Teflon tape (the kind you buy for plumbing work at the hardware store). I also added a couple of layers of electrical tape (600V rating) for additional safety. These modifications are shown in the picture below with the vertical tubing removed – you simply add the tape over the sleeve. The vertical element is then secured to the bracket by a pair of hose clamps of suitable size.

A construction detail shown in the picture below is the solid aluminum rod that fits inside the lower 14″ of the main 8′ length of tubing. The solid rod is inserted at the bottom to ensure a good tight connection for the sleeve. This rod end can be drilled with a blind hole for a self-tapping sheet-metal screw to secure the solid copper wire from the center conductor from the SO-239. The tubing is secured to the rod with a hose clamp just above the top of the bracket.

A tip for ensuring good clamping force with hose-clamps and hollow tubing is to slit the tubing about two inches up from the bottom on opposing sides with a hacksaw. This will allow the clamps to grip tightly enough to prevent slippage. Also, insert a solid piece of rod about 8″ long inside the smaller diameter telescoping tube at the top of the vertical to prevent that tube from collapsing. The upper telescoping tube is adjusted to about 15′ overall length to give proper loading across all bands.

The final photo below shows the completed vertical attached to the railing with the coax looped about 6 times to give some measure of RF choke action to keep RF from entering the shack on the braid. I secured the coax loops with plastic wire-ties to the railing support to stress relieve the connector. You can also see the tubing and small hose clamp just above the neoprene sleeve along with the two larger hose clamps gripping the sleeve and rod to the bracket.

It is quite easy to remove the vertical tubing element and stow it when you are not operating, as I now do, thus fulfilling the need for an unobtrusive HF antenna.

All in all, a cheap and effective radiator for the higher HF bands!

Article originally available at pages.zdnet.com/radio_k7zb/id8.html.

[tags]antenna,hamradio,dx,vertical,coax antenna[/tags]

This construction will take your 1-2 hours and it will cost you about $25.
cost breakdown below is for the material actually used, longer tubing lengths may be required that inflate the apparent cost.

Materials :
1 x 6.1 metre length 19mmx1.5mm round aluminium tubing ($12.75)
1 x 1000mm length 16mmx1.2mm round aluminium tubing ($1.50)
1 x 200mm length 38x25mm rectangular aluminium tube (x 1.0mm wall) ($1.80)
4 x 12-23mm stainless steel worm-style hose clamps ($1.50 each)
2 x 16mm (tubing size) plastic chair tips ($0.70 each)
16 x aluminium pop rivets
50 ohm coax cable, eg RG58A/U, minimum length 3-4 metres
200mm x 32mm white outdoor conduit
Nylon cable ties etc…

Calculated dimensions @ 50.1 MHz
Long section : 4290 mm
Short section : 1423 mm
Feed point spacing : 140mm (external coax ‘Y’ style) **
Element spacing -metal tube outer to outer at closest dimension : 135mm

** This dimension is based on the original ‘Y’ external feed. Using a modified feed system, this distance is about 180 -210mm. Details of the modified feed are listed near the end of this page.
 

Construction :
Creating the shorting stub, this is the hardest part of the entire construction :

The critical dimension is the 135mm spacing between the elements but this dimension is not the centre-to-centre value, it is the spacing from tube outer to tube outer.
With 19mm diameter tube, adding 19mm gives the centre-to-centres of the holes as 154mm.
Therefore the outside to outside is 173mm so it does not leave much from our 200mm material.
Measure in 23mm from one end along the 25mm side section and then drill a pilot hole as a guide for a larger drill.
Making sure it is square, mark the hole position on the 25mm section opposite face and drill it.
Enlarge the hole to 19-20mm. I cheated here “I used a chassis punch” from each side to get a neat hole with about 0.5mm of play.

From the inner edge of the hole, measure and mark the position 145mm along the tube on one face and then repeat the process for the opposite face.

Drill these pilot holes and enlarge them to 19-20mm.

Not really hard unless you don’t have a suitable drill press with a drill or chassis punches to get the neat 19-20mm holes !

The separator : To provide mechanical rigidity, an insulating separator must be fitted near the top of the matching section joining both tubes.
It is similar in dimension and technique to the shorting stub but is made out of PVC electrical conduit rather than aluminium.

Use the same dimensions as in the construction of the shorting stub for the spacing of the hole centres and drill 19-20mm holes through the conduit
(the chassis punch works well on the PVC tube too). Make sure that the alignment of the holes is correct otherwise the aluminium tubes will not fit through.
The separator is held in place by nylon ties fed around the vertical aluminium tubes but placed within the end of the conduit and later tightened so as to not allow any slippage down the aluminium tubing – and without requiring additional holes.

The round tubing now needs to be cut to length and prepared for fixing :

From the 6.1 metre tube length, cut off 1300mm – leaving a 4.8 metre section for the main vertical radiator and mounting.
Cut a contraction slot in one end of each tube (4.8m & 1.3m) section for about 50mm.

Mark 4000mm from the slotted end of the main radiator. This is the position for the main shorting stub.

Assembly into the finished product :

• Slide the shorting stub along the main radiator tube until the upper edge lines up with the 4000mm mark (the stub will be on the shorter length side of the mark).
• Making sure that the tubing is square to the cross stub, drill clearance holes in the 38mm section sides for the pop rivets you are using and set the rivets. I recommend drilling one hole and installing the rivet before drilling the next hole etc.. Rivet both sides of the bottom stub tube to the main radiator tubing. This will pull in the 25mm dimension of the tube a little but that is the reason why it is so close to the end of the rectangular tube !
• Install the unslotted end of the matching tube ? the shorter length of 19mm section into the shorting stub with about 3-5mm protruding through the bottom.
• Making sure that this tubing is also square to the cross stub, drill clearance holes for the pop rivets you are using and set the rivets. As before, I recommend drilling one hole and installing the rivet before drilling the next hole etc.. Rivet both sides of the bottom stub tube to the matching section tubing.
• Slide a worm clamp down each 19mm tube down to the matching stub.
• Form a loop from a single nylon tie making sure it will fit over the 19mm tube (make 2 of these.)
• Fit a looped tie into one end of the separator and carefully feed onto the main radiator tube
• Slide the separator down over the long tube first then over the matching tube so that it is just above the slotted top of the matching tube. Install the second looped nylon tie into the other end of the separator tube and feed it down over the aluminium matching tube until it is just below the slotted section. Pull each tie tight so that it will not slip on the aluminium tubing. Multiple ties can be fitted if desired.
• Slide a second worm clamp over the slotted section on each 19mm tube and insert the 16mm tube and adjust the worm to just hold the clamp in position.
• Measure the main radiator tube from the top of the shorting stub and set the adjuster to 4290mm before tightening the worm again to just hold the tube in position. Do not overtighten at this stage.
• Measure the matching stub tube from the top of the shorting stub and set the adjuster to 1423mm before tightening the worm again to just hold this tube in position. Do not overtighten at this stage.
• From the shorting stub, mark a position 140mm up each 19mm tube. This is the position to attach the coax cable. Note that the coax inner goes to the main radiator while the outer goes to the matching stub tube.
• Slide the worm clamps up to these marks and lightly tighten in position.

Final steps : Cable connection and adjustment – standard ‘Y’ cable feed.

• Create an RF choke by winding 5 turns of the RG58 cable around a 125mm former and nylon tie it into a stable structure. Leave at least 400mm of cable free from the feed end. The other end (the tail) can either be terminated in a BNC or other coax connector to suit the cable type or can simply be the start of the feeder that takes it to the radio.
• Strip back 100 – 125mm of the outer on the feed end.
• Push the braid back a bit to loosen it up, then poke a ?hole? in the braid and ?fish out? the coax inner.
• Strip the inner back to about 75mm of exposed poly and the rest the inner conductor.
• Tin the inner conductor and then screw it under the worm clamp on the main radiator.
• Feed the braid under the worm clamp on the matching section and pull it until there is just a little slack in the cable. Cut it off, remove it and tin the braid before placing it back under the worm clamp.
• Mount the antenna vertically in a clear space in such a manner that it can easily be brought down to adjust at each of the following stages :
• Connect a suitable transmitter via a SWR bridge to the coax ?tail? and check the VSWR at say 50.160 Mhz. Do not use 50.110 MHz as a test frequency !
• Check the VSWR at various spot frequencies in that segment. If the VSWR goes up as the frequency rises, the tube lengths are too long and need to be shortened. If the VSWR goes down as the frequency rises, the tube lengths are too short and need to be lengthened. Lengthen both together but make sure that the main radiator adjustments are about triple the matching section?s length changes.
• Once the VSWR is minimum at the centre of the desired section of the band, it is time to adjust the feed point positions on both tubes to bring the VSWR down to the absolute minimum. Slide the clamps up or down the tubes TOGETHER and see where the trend goes.
• If the VSWR rises, move in the opposite direction. It should be possible to get the SWR down to 1.05 or better without any problem.

Final steps : Cable connection and adjustment – modified cable feed.

• Drill a 5/16″ hole about 200mm from the top of the shorting stub along the inner side of the matching tube(facing the main radiator tube) and remove any burrs.
• Feed the RG58 up the short tube and pull it out of the hole.
• Strip back the cable outer sheath for about 200mm.
• Push the braid back a bit to loosen it up, then poke a ?hole? in the braid and ?fish out? the coax inner right at the start of the exposed braid.
• Cut the braid off with about 25-30mm left and tin it with a soldering iron making sure you do not overheat the poly inner of the RG58.
• Slide the cable back through the hole and set the worm clamp to hold the braid on the tube adjacent to the feed hole.
• Feed the coax inner across towards the main radiator element making sure you have about 20mm of slack on the poly and enough inner to tin about 30mm before cutting it off.
• Feed a length of the black plastic coax sheath back over the inner to provide some protection against UV.
• Clamp the inner to the main feed directly opposite the feed hole as an initial location.
• Create an RF choke in the coax by winding 5 turns of the RG58 cable after it exits the tube around a 125mm former and nylon tie it into a stable structure and nylon tie it to the base of the matching section. The tail can either be terminated in a BNC or other coax connector to suit the cable type or can simply be the start of the feeder that takes it to the radio.
• Mount the antenna vertically in a clear space in such a manner that it can easily be brought down to adjust at each of the following stages :
• Connect a suitable transmitter via a SWR bridge to the coax ?tail? and check the VSWR at say 50.160 Mhz. Do not use 50.110 MHz as a test frequency !
• Check the VSWR at various spot frequencies in that segment. If the VSWR goes up as the frequency rises, the tube lengths are too long and need to be shortened. If the VSWR goes down as the frequency rises, the tube lengths are too short and need to be lengthened. Lengthen both together but make sure that the main radiator adjustments are about triple the matching section?s length changes.
• Once the VSWR is minimum at the centre of the desired section of the band, it is time to adjust the feed point positions on the radiator tube to bring the VSWR down to the absolute minimum. Slide the clamp up or down the tube and see where the trend goes.
• If the VSWR rises, move in the opposite direction. It should be possible to get the SWR down to 1.05 or better without any problem.

When all is done, the centre frequency is as desired and VSWR is negligible, it is time to put the plastic chair tips on the tops of the tubes, tighten the worm clamps and weatherproof as desired. Don’t forget to weatherproof the slotted adjustments and their worm clamps with a marine varnish to prevent excessive oxidisation. If using the modified feed, remember to seal up the hole where the coax feed exits across to the main radiator with a good silicone sealant.

Final dimensions at 50.1 MHz

Long section : 4425 mm
Short section : 1513 mm
Feed point spacing : 160mm
Element spacing : 135mm

This project was originally edited by VK4ADC, but this page has been removed, luckily I did saved a copy in my disk and this is a simple extract. All copy rights to the original author.

[tags]antennas,j-pole,hamradio,amateur radio,50 mhz,dx,antenna[/tags]

This small antenna can allow hams which lack space to install an antenna for 40 meters. This project has been originally  produced by F6CYV. I’m going to test this antenna in the coming weeks. I will try to setup this inside my balcony.

According to his experience, using it form inside the apartament, european singals are all very readable, he has worked over 150 countries.

The antenna is made of 2mm wire.

The 2 coils are constituted by 18 turns of 2 mm wire, distance of tunrs is also 2 mm.

The diametre of the coils is of 7,8 centimeters.

The Feed of the dipole is done with a 75 ohms tv coaxial cable.

A 1/1 balun would be recommand for a correct feed of the coaxial cable to the dipole.

It is not necessary  to use a coupler, it is enough to set the length of both extremities of
the dipole in order to have at 7.050 mhz a low SWR, and especially to pay attention what the lenght of the 2 sides of the dipole to be identical.
[tags]antenna,ham-radio,amateur radio,HF antenna[/tags]

G5RV verses Superloop 80

Many operators with small lots, a G5RV is what can fit for the 80 and 40 meter bands. The G5RV is 102 feet long and has a 34 foot
section of twinlead followed by coax into the shack, possibly with some sort of RF choke on the coax. The ends are typically supported by ropes up in
the trees. An 80 meter dipole would be about 134 feet long.

A tiny lot is limited in antenna potential and zoning laws prevent real towers.

RadioWorks “Superloop III” designed by Jim, W4FTU, and refined over the years, is a good alternative

PHYSICAL VARIATIONS

The standard arrangement is shown in Fig. 1. It looks like an inverted delta loop and is 112 feet across the top. It fit on the same ropes as my G5RV used and the coax even started at about the same point in space. The wire is heavy 14 gauge copper. If your space doesn’t quite allow this, the top corner insulators can be moved to shorten the 112 foot dimension; also additional insulators can be added to the diagonal wires to make a rectangular
shape and raise the bottom balun up in the air more. I also added 6 feet of wire to move the resonant freq closer to the band bottoms for digital work.

The loop can also be mounted upside down and slanted if you only have a single support available. As with all loops, the area enclosed is important and so is the average height; the standard inverted delta shape is a very good compromise.

ELECTRICAL CHARACTERISTICS

The “trick” to the Superloop is the 30′ length of ladder line hanging down from the center insulator. This length has been tuned so that appears to be a open-circuit stub on 40 meters; thus the antenna becomes two full-wave wires (at 40 meters) and is commonly referred to as the Bi-Square antenna. On 80 meters, it appears to be a short and the antenna becomes a single wave vertical loop. This happens automatically and no switching is involved.

A special balun is provided which gives a match between the 50 ohm coax lead-in and the higher resistance of the loop. For best matching, a 1/2 wavelength coax is recommended (e.g. 99′ of RG-8X); however mine is about 70 feet into my diff-T tuner and the SWR < 2 points are 3495 to 3787 but the short coax gives a minimum on 40 of 2.05 at 7090 KHz. If you need to run without a tuner, close attention to the coax length will help. The balun is the typical ferrite rod in a PVC pipe with foaming urethane inside. This has the effect of heat insulating; mine works fine on 500 RTTY watts contesting, but real high power may be a problem on RTTY; but those guys all have beams, right?

OPERATING RESULTS

The diagonal wires make it partially a vertical antenna with a nice reduction in polarization QSB. You can possibly double contacts on 80/40 over the G5RV. RITTY can help on the reception. The Superloop tunes up fine on the 20,15,10 bands Antenna, ropes, and coax will run you about $US 135. RadioWorks advertises in CQ and QST and have an interesting catalog.

Copyright and originally hosted at http://larc.hamgate.net/SuperLoop.htm

[tags]antenna,ham radio,amateur radio,loop antenna[/tags]

As much as I like my coax loops, I am also quite satisfied with small loops made with wire or tubing. They have the same or better performance as the coax loops, but might require that you invest in a balun to help maintain directivity and avoid common-mode noise ingress from the feedline. If you need to null local noise yet still be able to listen to most skywave signals, these loops really perform.

The antennas described below bridge the gap between operating as a constant-current small loop (0.10 wavelength or less circumference), and intermediate-sized loop a bit larger than 0.17 wavelengths long in circumference.

If you are interested in building loops made entirely from coax cable you may want to check out my earlier project pages on that subject. It has many operational notes and other items of interest that pertain to small plain wire loops as well as to coax types.

The voltage balun was essential to help me fight common-mode noise and maintain directivity. If you don’t use a balun and have good results, you may not have much noise to deal with in the first place, or the skewed directional pattern has a null that works for you – even if it isn’t textbook. See my balun notes below.

I initially chose 14 feet since my noise problem extends up into the 40 meter band; I didn’t want the antenna to be longer than 1/10th wavelength because you start to lose your nulls with larger wavelengths of wire. I just did a quick calculation: (1005 / 7.150 * 0.10)

Note that I have since opted to use 28 feet overall, because I wanted better sensitivity on 160 and 80 meters, and now at 40 meters the 28 feet of wire still gives me a slight null – adequate enough for me to null my local noise on 40. Unfortunately I don’t have the room for a full-sized loop, so I had to wind it with two turns. See the EZNEC® antenna modeling plots below.

Here are some quick construction tips to get you up and running quickly. I’m still studying the antenna and will improve the page as time goes on.

I am running the loop right at the operating position. Here is what I’m using:

• 14 feet of #12 gauge wire formed into a loop. Coax braid is an option.
• W2AU 1:1 voltage balun by Unadilla.
• 10 foot coax jumper from loop balun to tuner input.
• Common tee-type C-L-C antenna tuner.

The loop seems nearly omnidirectional for medium to high-angle skywave signals, yet has great noise-nulling directivity at very low angles from 160 – 20 meters. These two qualities make it a great general purpose antenna especially indoors.

Let’s take a look at the elevation angle for 20 meters. It shows good medium to high-angle skywave directionality. The other bands have much the same elevation pattern:

Look at the azimuth angle for 80 meters. 160 and 40 meters are similar. Notice the deep null; great for nulling noise by rotating the loop:

At 20 through 10 meters, the circumference of the loop is becoming progressively larger than 0.10 wavelength, and starts to have a nearly omnidirectional horizontal plane no matter how you rotate it. Fortunately I don’t have to null out any local noise on 15 and 10. On 20 meters I have a very minor noise problem, and the smaller null on 20 meters takes care of it.

This means that this 14-foot circumferential loop is performing as a small directional loop on frequencies of 40 meters and lower, and as an omnidirectional intermediate-sized loop on bands higher than 40.

The most efficient small HF loops are single-turn affairs. Multi-turn loops of this type are less efficient, but you may have no choice to wind a smaller loop with multi-turns if you can’t find the space for a single-turn, such as with indoor applications.

If you are really space constricted, you could cut the dimensions down and run a 7-foot circumference loop. Just don’t expect great performance on 160 or 80 meters. Rectangular loops might also be considered if you have a lot of vertical or horizontal space, but not much of both at the same point. Perhaps you have very high vaulted ceilings in which you can make long vertical runs for the sides of the loop whereas the horizontal runs would be much smaller.

Just remember that the key to small loop success is to enclose as much AREA as possible; keeping in mind that when your antenna starts to appear longer than 0.1 to 0.25 wavelengths in circumference, you’ll start to lose the deep nulls.

Strive for a single-turn loop, but if you must, you can wind multi-turns if you have to. To help reduce the proximity effect of the turns, (one of the elements of loss resistance) try to keep the multi-turn loop wires spaced one or two wire-diameters apart.

Skin-depth rf currents on closely spaced coil conductors have a tendency to reject each other and “pool up” on opposite sides of their respective wires, thus effectively reducing their own conductor area. I have had success by winding one turn on one side of my pvc mast, and the other turn on the back of the mast. However, I am not so sure that this is very critical for a receive-only application. More study required …

Balun experiments:

I have had the best results using a VOLTAGE balun at loop feedpoint. I tried a hefty commercial 1:1 CURRENT balun, and it turned my small loop into a noisy non-directional random wire.

(I’m not condemning the use of choke or current-type baluns, it’s just that the voltage-type balun seems to work better for me in this application.)

Since I wanted my loop to be general-purpose and work across several bands, I made no attempt to match the loop impedance to the feedline. This may be affecting balun performance somewhat, but so far it is performing adequately with the loads presented by the loop in this rx-only application.

I really didn’t want to use a balun, but found that I had to. I experimented with a direct connection to the coax without a balun, and got some directivity on 160 and 80 meters, but the common-mode cable ingress noise on 40 meters and higher was pretty bad. It also changed my tuner settings radically. Putting some clamp-on RF cable chokes on the feedline reduced the noise a bit. I reinstalled the 1:1 voltage balun, took off the chokes since they were no longer necessary, and got my deep nulls and quiet reception back.

Keep your balun connections neat and symetrical. For example, the 1:1 voltage balun I use is a W2AU type and it has small jumper wires behind the strain-relief eyelets. Connect the loop wires to the jumpers close to the eyelets, and then symetrically dress the remainder of the balun leads neatly. I made the mistake of letting the balun wires hang in a hay-wire fashion, and attached the loop leads to random points along the balun jumpers. Although the antenna worked well, tighter nulls and better overall balance was achieved just by being a bit neater with my connections. It’s worth the effort.

What about a 4:1 voltage balun? It works well! On a lark I thought I’d try a 4:1 balun and see how much worse it would be. To my surprise, I still have my low-angle bidirectional directivity, my nulls are sharp all the way from 160 to 20 meters, and my tuner settings require about half the inductance! (except on 160 where my inductor settings stayed the same). It seems that as long as I can tune out the reactance of the antenna system, the loop-to-feedline mismatch isn’t as much of a concern as I once thought. I can’t even begin to explain what’s going on with all the variables. I’d sure like to learn how to model small loops with differing balun ratios … until then I’m enjoying the loop with the 4:1 balun.

Quick Solutions:

I had a 50-foot piece of coax left over from my earlier coax loop experiments, so I thought I’d experiment with it by using just the braid as the antenna element (continuous braid loop – no gaps). I had to take into consideration noise-nulling vs sensitivity for my location. I prefer single-turn loops, but in some cases I had to wind them into multi-turns (with one wire-diameter spacing) to fit indoors. In all cases I used my tuner to resonate the whole system. The lengths listed below are not super-critical.

The first band listed is operating as a 0.05 wavelength loop, and the second listing is operating as a 0.10 wavelength loop.

160 – 80 meters optimized: 52 feet of wire

This is the best 160 meter loop I have used to date. To fit indoors, I had to wrap it into 4 turns. Deep nulls on 160, medium nulls on 80, everything higher in frequency turns omnidirectional. I really wish I had the space to open this up as a single-turn loop, but I have to make do with the space I have.

80 – 40 meters optimized: 26 feet of wire

I still have a noise problem on 40 meters, so I had to cut the length of the wire down to get my deeper nulls back. I still had to fit it indoors by wrapping it with 2 turns. Deep nulls on 80, medium nulls on 40, everything higher in freq omnidirectional. (I can still copy the locals on 160 ok, but since the loop is smaller than 0.05 wavelength on 160, it’s very inefficient. Considering that the loop is best for medium-to-high angle skywave reception anyway, this isn’t as bad as I thought on 160. So what if the locals on 160 are a bit weaker – the SNR of the loop makes it usable anyway.) I’ve also reduced the top-heavy weight of the loop by using less turns. This is now the favored loop size for my situation. In this case, less is more!

40 – 20 meters optimized: 14 feet of wire

This wire length now allows me to use a single loop of wire. Great nulls on 40, medium nulls on 20. Everything higher in freq omnidirectional. Since I don’t have a big noise problem on 20, I use the larger loop in the previous experiment. The loop is so light with only one turn that I’d probably use a much bigger conductor diameter for the loop if I wanted to maximize sensitivity on 40 meters.

Velocity Factor issues with outer coax braid (basically none!)

Since I’m using the outer braid skin of the coax as the antenna element, I can ignore cable velocity factors. In other words, the velocity factor is only applicable to the inner differential-mode currents, and not to the common-mode current that exists on the outer braid skin.

I’d like to offer my apologies to earlier readers where I indicated that the velocity factor of a loop using coax braid as the antenna element should taken into affect. I was wrong.

Wire diameters for 0.10 wavelength or smaller loops
I recommend using 1/4 to 1/2-inch diameter or larger conductor diameters for the loop if you desire them to be self-supporting. You can also use just the braid of RG-58 or RG-8 coax and affix it to a mast.

Small-gauges of wire don’t perform as well as tubing does with loops under 0.10 circumferential wavelength, and larger conductor sizes, while offering greater performance and a larger bandwidth, may present a problem when considering the cost, weight, and general hassle of construction. The general rule of thumb would be to use the largest diameter conductor that you find practical so that you can lower the loss resistance.

Tuner Notes
Since small loops are high-q antennas, it is very easy to mis-tune or mistake a peak in your tuner settings for an optimal match of the system. If you have a noise-bridge, or antenna analyzer handy, this can make finding the right tuner settings much less of a chore. I’m going to describe doing it “by ear”.

After building a new loop I usually get impatient and madly start adjusting the caps and inductor settings hoping that I can hear the peak quickly. Sometimes I get lucky, but more often than not, I dont’ find any peaks, or I end up on a very inefficient one. Sadly, I resign myself to the fact that I’m going to have to do it in a more methodical fashion and maybe eat up an entire afternoon to find the settings for most of the bands.

Let’s assume you have a typical C-L-C type tuner; a cap for the receiver side, an inductor, and a cap for the antenna side. You’ll want to do this when the band is open, or perhaps tune to a local noise source. Here is my generic method going from the lowest to highest freqs (tedious to be sure, but I don’t want to accidentally skip over a great match):

1. Set L for for maximum inductance.
2. Set both caps to zero, either fully meshed or fully open.
3. Set the antenna cap to 1
4. Rotate the receiver cap all the way through it’s range.
5. Set the antenna cap to 2
6. Rotate the receiver cap all the way through it’s range.
7. Continue with the above steps advancing the antenna cap by one.
8. If no peak is found, lower the inductor value, and try the caps again.

On the lower freqs, you may even want to advance the antenna cap settings by only half-steps until you find the peak. Ugh.

Even if you do find a nice peak, don’t give up just yet! Make a note of the settings, and try it again with the next inductance value. You might be surprised at how well the NEW match point works. This is pretty tedious stuff, but the result are worth it – don’t be tempted to be satisfied with the first peak you find!

After you’ve found the major match settings, you’ll probably need to make slight capacitor repeak adjustments when you tune the receiver from one band edge to the other, especially on the lower 160 and 80 meter bands.

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