Mars Cap and freeband modification for the VX-7R transceiver
I recently had a conversation on the radio with another Ham who had built a 6 Meter wavelength band “J” antenna. He was getting some rather strange performance from this design, so I asked him where or how he had come up with the antennas dimensions. He referenced an Internet web page to me where I learned a calculator was provided to obtain dimensions for any desired frequency. I was immediately suspicious!
You see, the dynamics of antenna performance are such that “rounding errors” when scaling an antenna will basically destroy an antennas performance, dependent upon the relative frequency, or actually the percentage of wavelength. It works like this, if we choose some abstract “Constant” or “K factor” to design an antenna, that will work — over only a relatively narrow range of frequency.
This becomes particularly troublesome when the K factor is developed at some relatively high frequency, such as the 2 Meter wavelength band, and then it is used to scale antenna dimensions to lower frequency bands — such as 6 meters or 10 meters. If we used this same technique to scale up in frequency, it would work better, or at least we would not as likely notice any negative results. Here is why.
At a frequency of 146 MegaHertz (MHz) a piece of 3/4 inch diameter pipe is .009271 wavelengths in diameter. At 51 MHz., this same pipe is .0032385 wavelengths in diameter, or only about 1/3 the wavelength diameter. On 223.5 MHz. this same 3/4 inch diameter pipe is .0141922 wavelengths in diameter. You see on the 222 MHz. band this pipe diameter is a pretty big fraction of the wavelength; pretty fat. At 50 MHz. though its pretty skinny! It would look even skinnier at 29 MHz., and consequently have very high impedance, or AC resistance, at this lower frequency!
The larger fatter diameter conductor, relative to frequency, has greater bandwidth, and generally better performance overall. At relatively lower frequencies its skinnier percentage of wavelength makes its performance narrower in frequency response, and we will see general or even dramatically degraded performance. If in addition to this our numbers are not really exact, or are compromised by rounding errors, the performance will be markedly bad!
You can try this out. The constants or K factors for my 6 Meter “J” antenna are *1 8161.5625 for the radiating elements overall length. The K factor for the Q-line is *1 3023.75. These constants would be close enough to allow a “J” antenna to be designed over any part of the 6 Meter band, and probably for a megacycle or so above or below this band. Try to use these same constants to build a 2 Meter wavelength “J”, and also one for 10 Meters. See what happens!
Terms used in this article
Percentage of wavelength: Given conductors, be they pipe, rod, tubing, or wire, represent a relative percentage of the wavelength of the antenna operating frequency. Larger fatter conductors (relative to the operating frequency) have greater surface area, and consequently lower AC resistance. This gives them greater bandwidth, and better efficiency in their radiation performance.
Constant or K factor: A commonly known K factor or constant is the one used to determine the length of a half-wavelength center fed dipole. This constant is 468 / f in MHz. and has appeared on every FCC Ham license exam for better than 30 years. Other constants that can be developed will allow antennas or other constructs to be fabricated from existing designs. Rounding errors, and dynamics of frequency conversion must however be accounted for! You can’t maintain only certain factors such as tubing diameter, or element spacing, and not have them effect the whole!
Scaling: Antennas can be mathematically scaled up or down over relative wavelength but, all factors must be considered uniformly, or errors will accumulate.
Beyond this I would suggest a book written by the preeminent antenna engineer John Kraus W8JK. His book, “Antennas” (ISBN 07-035410-3) published in 1950 is the exalted antenna compendium for nearly all antenna design work, commercial or Amateur. It explains all of the physics associated with how and why antennas work.
*1 I worked out these numbers only for sake of this article! I did not originally use any calculation to build my original design, which I did about five years ago. I built that antenna from approximations, then I dialed its performance in from empiric tests. Now that I know dimensions that work in this frequency range, I can work backwards to develop these “K factor” numbers.
Article by Wa6BFH originally at /www.geocities.com/SiliconValley/2775/
Frequency Filer is database program designed for people who use a great many different radio frequencies. It is primarily intended for amateur radio operators, shortwave listening enthusiasts, and scanner users. The program makes it easy to keep all of your important frequency information in one place. Geographic information can be displayed on a high resolution globe. Includes data conversion utility.
Shareware by Joel Graffman
FIG. 1 shows the relative gain (loss) of an antenna (e.g., dipole or beam), under varying ground conditions, through the frequency range of 5-30MHz:
A represents the curve under perfect ground conditions, B under average ground conditions (i.e., wet grassland), and C under poor ground conditions (dry desert).
Note that the influence of ground conditions diminishes rapidly as the frequency increases.
While a good ground system can substantially improve performance at the lower frequencies, at 30MHz there is very little difference in gain between a dipole with a perfect ground and one with poor ground.
For example, at 5MHz a receiving or transmitting signal that is S9 +10 dB over perfect ground becomes S9 +7 dB over extremely poor ground (a 3 dB drop); but at 30MHz, a signal that is S9 +10 dB over perfect ground, drops to S9 +9.4 dB over extremely poor ground (a drop of just .6 dB!).
Conclusion: The influence of ground conductivity becomes less important at 14MHz and higher (1.2 dB difference or less).
A typical 3-element Yagi 1/2-wavelength high over perfect ground has approximately 14 dBi gain (as compared to an isotropic radiator in space).
Information from “Neues von Rohde und Schwarz” Oktober/Nov. 1973.
This article reports the results of searches performed on the G5RV antenna. As stated in several articles concerning the G5RV antenna an specially in old French books written by R.A. Raffin, F3AV and R. Piat, F3YX and more recently. Unfortunately years have passed and I only kept in memory that at the bottom of the line the impedance has a small reactive part and a 75ohms resistive part then any length of 75 ohms coaxial line can feed it.
Today , as regards what I heard on air it is always used with an ATU, mainly on 80, 40, 30 and 20m. I think it’s partly due to the fact that the solid state PAs don’t allow any impedance adaptation when in the past tube PAs had an output circuit tolerating more or less a load with a reactive part. On 20m and higher it seems that commercial multiband antennas needing no settings have superseded the G5RV antenna.
The G5RV on internet
It provides us with several KE2DI articles about the G5RV antenna. They are the most interesting I was able to find because they provide original information and also today consideration. As a sum up, the features are:
q The antenna length is 31.10m (102ft), so it’s a 3/2-wave on 20 m and installed horizontally at 12m (39ft), the resonant frequency is 14.150MHz and the resistance is about 80ohms.
q On 20m and up the gain is better than a dipole because of the presence along the wire of several 1/2-waves, which give a collinear effect.
q The 10.36 (34ft) stub line length is designed to be a 1/2-wave on 14.150MHz and on the other bands it acts as a useful impedance transformer.
q The aim was to get a multiband antenna without traps and for limited area
Applying a 0.97 velocity coefficient (air ladder line) to the 10.36 m (34ft) of the stub line give exactly an electrical 1/2-wave on 14.150Mhz. Therefore, the stub is a 1:1 transformer on 20m, so on 14.150MHz there is at the bottom of the line the same impedance as at the antenna center.
Today in France a lot of hams use this variant. The difference lies in using a 1/4 current balun at the bottom of the 10.36m (34ft) stub line where it ensures the transition symmetrical to asymmetrical. This balun configuration is named remote balun.
Flat-top installationTheoretically to be perfectly symmetric the two antenna wires must be aligned in the same horizontal plane and not threw out of balance by environment masses. If it is not the case there will be unbalanced currents in the conductor line, it will radiate and will influence the antenna. Also the collinear effect will be decrease
Inverted-V installationTheoretically to be perfectly symmetric the two antenna wires must be straight in the same vertical plane and not threw out of balance by environment masses. If it is not the case there will be unbalanced currents in the line conductors, it will radiate and will influence the antenna. Also the collinear effect will be decrease. If the apex angle becomes lower than 120°, the benefit of the collinear gain on the highest bands will be progressively lost. It means that if the apex angle is 12m (39ft) high the wire ends must not be lower than 4.25m (14ft).
I haven’t yet built up a G5RV antenna in order to check the above features, which are obviously theoretical. However I have carried out some tests with a 2x10m center-fed antenna fed by a 450 ohms twin-lead and a 1/1 balun connected to the asymmetrical out put of my ATU, see remote balun, so:
q An ATU is required to match the various impedance got at the end of the stub
q The ATU can meet its matching goal on some band but possibly not on some other
q The ATU cannot suppress the SWR in the coaxial line and consequently the losses as well
q If a balun is used there will be more losses on some band
q Although the original article tell that it is conceivable to use a balun, to day on the web it is recommended not to do so, of course it is understandable that the wonderful balun doesn’t exist.
q If like me you hate to waste power in a loss-making system of antenna feeding, the solution to run the multiband properties of a 3/2-wave-20m is to use an ATU with symmetrical output. The symmetric line length required is no longer 10.36m (34 feet), but it must be optimized as regards the ATU possibilities. Even if it is right that the line work with a very high SWR, due to its conception a parallel conductor line generates tiny losses in standing wave rating. But it’s no longer definitively a G5RV antenna.
The information in this article has come from many amateur sources, the most notable was from WA6TEY (sk 1985) Ray Frost, who was a pioneer of VHF Quad designs and one of the best Southern California Transmitter Hunters of the 1980’s. Ray built hundreds two meter quads in single and paired configurations as well as his famous mobile radio direction finding quad. At Ray’s request I have used his information and expanded upon his basic designs.
Dimensions for the inverted V antenna from 160 to 2 meters by N6JSX
This modification has been found on the net. Please use at your own risk.
WARNING: Improperly performed modifications can severely damage your adio. I have performed these modifications successfully, but I offer no guarantee or warranty for them. Proceed at your own risk.
1) Small philips-head screwdriver
3) Magnifying glass
4) Low-wattage (15 watt) soldering iron
5) Long-nosed pliers
Enables out-of-band transmit for 1.6 MHz to 54 MHz.
This does not enable extended VHF transmit This does not enable AM or FM broadcast band ransmit. Your memories will be cleared after this modification, ince you need to reset the CPU.
1) Open the top of the radio by removing the 3 top screws and 2 side screws. Look at the radio from the with the front panel facing you. crews.
2) Gently pull up the speaker and set is aside without damaging the speaker or the wires that attach it to the rig.
3) Note the silver rectangular box near the middle of the PCB marked something like “9 MHz SSB Filter”.
4) Move your eyes up from this filter toward the back of the radio.
Just before you get to the “D 108” marking, you will see two tiny diodes, two blank spaces, and one additional diode. They look
something like this:
[XX] [XX] [ ] [ ] [XX]
Remove this diode ^^^^
5) The second diode from the left needs to be removed. I did this by crushing it with long-nosed pliers. You can also heat it with a
low-wattage soldering iron and pull it up with tweezers. Be sure not to damage the other diodes or the PCB. Be sure that you don’t
apply too much heat, since the heat can damage the PCB and the other diodes.
6) Re-assemble the radio. Reset the CPU by pressing and holding down the UP and DOWN buttons on the front panel and pressing POWER.
Improved VHF recieve mod:
154-200 MHz RANGE
To open up the 154-200 range,cut the yellow wire on the plug labled “j-4” on the right side of the radio on the bottom,the book points to this connector.
I did this and it opened up the rx between 154-200 mhz.
(It was printed in CQ VHF a couple of months ago.)
For those who like to scan VHF Hi-band, there is a mod that restores sensitivity, but at the cost of reduced sensitivity below 120 MHz.
This allows the filter to switch at the corner frequency (apparently around 129 MHz) as you tune.
PROCEED AT YOUR OWN RISK
There are no guarantees you won’t trash your radio.
Remove the radio top and bottom covers according to the manual.Remove the speaker. Now with the radio facing you and right side up, you’ll see a single connector with discrete wires at the rear of the control board. The fourth wire from your left should be a yellow wire (caution, the 5th and 6th wires on the other end of the connector are yellow too, don’t get confused).
Disconnect this wire from the connector. This will enable the 2 meter band-pass filter to work outside the 2 meter band, and will restore sensitivity between about 130 MHz and about 165 MHz (above and below that range, sensitivity still stinks, especially above 165 MHz). Before this mod, a low pass filter with a corner frequency around 129 MHz was in the circuit all the time except when you were actually tuning inside the 2 meter band, and that killed high band sensitivity completely. Before removing the yellow wire, sensitivity at 155 MHz was 30 uV for an S1 indication,after the yellow wire is removed, sensitivity increases so that only 0.5 uV is required for a S1 reading. At 165 MHz after the mod,sensitivity is 3 uV for an S1 reading, before the mod, a very large signal was required for an indication.
Disconnecting the yellow wire has the unfortunate side effect of reducing sensitivity between 60 and 129 MHz since the radio never switches from the 2m bandpass filter to the low pass filter. Signals below 60 MHz don’t go through either filter and are unaffected by the mod.
Now here’s how to get the low pass filter back when tuning below 129 MHz. On the bottom board, right behind the MENU button,
there are 5 SMD transistors. These transistors apparently switch the VCOs for the various band segments. The one in the middle of the 5 apparently switches the 60-129 MHz VCO. The single pin (one side of the SMD transistor has 2 pins the other only 1) switches to 5 volts when this VCO is active, and is low otherwise. This is exactly what we need to feed the yellow wire. There’s a board trace coming from this pin over to a feedthru hole near J8. Connect the yellow wire you disconnected earlier here.
The filter should now toggle between low pass and 2m bandpass as you tune below and above 129 MHz, and you should have good sensitivity both above and below this frequency (at least up through 165 MHz).
Remember, CAUTION WARNING This is tiny SMD stuff. If you trash your radio, you’re on your own.
As far as FM Broadcast intermod in the Aircraft band, I noticed the IF is a little overdriven into compression, so I turn the preamp off(greeen to no light—preamp switch), and noticed no difference in sensitivity, intermod in aircraft band disappeared.Running the preamp in the Aircraft 118-129 does not really help sensitivity, even though the S-meter shows higher signal levels (jumps around alot due to saturating IF when signals are not there!!! )
the noise floor actually, degrades, thus I leave the switch (no preamp-black instead of green). Leaving it on green is just driving the IF into saturation, with worst dynamic range.
Now connecting it to the VCO switch bank, the radio performs to my satifaction, hearing the weather at 162, forestry at 171, TV audio near 200Mhz and remembering to turn the preamp off in 118-129 aircraft region, no 2 meter images any more in 82-83Mhz area from 2 meters.
I can listen to FM broadcast in the 88-108Mhz area.
Q:Does this mod affect HF or 6m?
A:The mod affects only frequencies above 60 MHz. If you just cut or remove the yellow wire, the 2m bandpass filter is used all the time. After the mod you can listen to the airport on 134 MHz but 128.4 is still drowned in intermod from the FM BC band.
Proceed at your own risk
The receiver sensitivity above 120MHz (except the band between 144-148MHz) is very poor and also the transmitted FM deviation for NARROW FM is too small; therefore I did some tests and came to the following two modifications.
To carry out these modifications it is necessary to use the IC706 service manual, which can be bought at any ICOM dealer.
1. When choosing NARROW at FM, then during transmitting the max. deviation is reduced from 4.8kHz to 2.4kHz. It is still desirable to choose NARROW at FM because the receive performance is better then.
After changing resistor R272 from 1K to 8K2. (at the bottom side of the MAIN UNIT), then the max. deviation during transmitting goes from 4.8kHz to 4.3kHz when choosing NARROW at FM.
The modulation at AM is also changed now, but this can be corrected with potmeter R271.
2. To improve the receiver sensitivity between 120 – 144MHz and between 148 – 200MHz the following modifications can be carried out.
At the PA UNIT, change the 60 – 200MHz bandpass:
1) Remove C53(20p), C152(20p), C153(12p) and C154(20p).
2) Short-circuit L49(82nH) by soldering an interconnection at the place of the removed C153.
3) The inductance of L16, L17, L18 and L19 must be reduced somewhat.
This can be done by separating the windings somewhat with a small screwdriver.
Now the sensitivity is good up to about 175MHz. Also the sensitivity in the airband is much better now.
For receiving above 175MHz the low-pass filters at the ANT2 input have to be changed (components around L16, L17, L18, L19. I would not recommend that, because the spurious suppression during 2M transmitting becomes worse then!
To expand the band on the VHF portion.
(I have not tried this mod.I received this infomation via e-mail)
Caution: This is quite involved. If you are not too good you might be advised to get someone who has the experience to handle this mod!
1. Remove the main board from the unit. Keep the cutout hole away from you. This I consider the top of the board.
2. Remove the shield from the top of the board. There are 30-40 solder
points from the shield to the board.
3. Under the board on the top of the board, remove R-353 and Q-38. They are located to the left of the IC-36 chip on the top of the board under the shield.
4. On the underside of the board, Locate IC-32. There are two IC chips. IC-32 will be just down and to the right of the shield.
5. Using a pointer, point at the left upper pin, and go toward the top of the board. You will find a trace that stops. It comes out from under IC-31, and stops. It should be the sixth trace up from the top of the Chip.
6. Using a Xacto knife, (or something similar) Cut the trace the bend halfway between IC-31 and the solder point.
7. Make a jumper wire, and jump the connection, from Pin 11 of IC-32 to the newly isolated trace.
8. Reassemble the radio.
Transmit from 200 Hz to 200 MHZ continuous!!!
ICOM does not warranty these mods.
The mirror-image J-Pole is very easy to construct and requires only two T-Fittings plus a length of copper pipe.
To visualize the antenna, one must only picture a 3 half-wavelength vertical and a 2 quarter-wavelength vertical positioned a few inches away from and centered on the the tall vertical.
Construction of the mirror-image J-Pole is accomplished by starting with two T-Connectors separated by the distance shown for the band of choice. For two meters the space between the two vertical pipes should be 2 inches.
Two vertical pipes will point upwards and two vertical pipes will point downward from each T-Connector.
The tall vertical pipes are affixed to one T-Fitting and the short vertical pipes are affixed to the other T-Fitting. The tall vertical pipes are of the length shown as the Overall Length on the plans for the Copper Cactus. For 2-meters this distance is 58 inches to the centerline of the T-Fitting or an overall antenna length of 116 inches.
The short vertical pipes form the tuning stubs and are the length shown as the Stub Length on the plans for the Copper Cactus. For 2-meters this distance is 19-1/3 inches to the centerline of the T-Fitting or an overall stub-length of 38-3/4 inches. The nice thing about the mirrored-J is that mistakes in measurements up to 2 inches on the long vertical and 1/2 inch on the tuning stub, will not affect the performance or SWR of the antenna.
The mirrored-J is mounted on the end of 1-1/2 inch PVC pipe about 5 or 6 feet long. The PVC pipe is mounted horizontally from your tower or roof gable. The tall vertical pipe of the antenna is slid through a comperable size hole drilled into the side of the PVC at the end. A PVC T-Fitting may be sawn in half and used as a saddle to mount the antenna to vertical masts.
The antenna itself sits vertically. The coax is connected as per the instructions on the Copper Cactus plans to the upper vertical and tuning stub. On this particular antenna, the center conductor of the coax should go to the tall vertical and not to the tuning stub as in normal J-Pole construction. As always, keep the center conductor of the coax as short as possible, lengthening only the shield if necessary, using copper wire.
For Safety, the shield of the coax should be grounded to earth ground before entering your shack.
Article posted by KGØZP