VERTICAL DIPOLE

Since I had purchased the Kenwood TS-590 transceiver a few months before I got my license, I needed an antenna (or antennas) right away for listening on the ham bands.

The antenna that I put up first was a vertical dipole for 10, 15, 20 and 40 meters. The reasons for this selection as the first antenna to be installed are that

1. The vertical antenna has many devoted supporters in the ham community who claim to get fantastic results,
2. The vertical dipole has a very small foot print and requires no radials,
3. This vertical is self-supporting and is very easy to install (except for the many tuning adjustments that are needed) and,
4. It is almost invisible from the street view in front of the house.

This last reason was considered important for the first antenna as it would let the neighbors get used to antennas sprouting up on and around the house without being initially overwhelmed by some visually gigantic structure.

Two Hustler Model 6-BTV antennas were adapted to make up a half wave vertical dipole by mounting them butt- to-butt on a twelve-foot long 2 by 12 inch wooden mast attached to the deck railing at the rear of the house.

The center feed point of this combination is only about 22 feet above the ground.

Therefore, I had to eliminate the 75/80 meter sections of the antennas in order to reduce the total length for each side to about 20 feet.

This vertical dipole configuration was chosen because my house and back yard layout (a full coverage concrete patio was already installed) do not lend themselves to the installations of a good ground radial system.

One great feature of the way that this antenna is mounted is that the dipole can be rotated to horizontal and the ends can even be swapped – which makes it very easy to reach any of the tuning traps from ground level or from the deck to which it is mounted! In addition, I have heard some hams say that with an incline angle of 45 degrees the signals come in earlier and leave later as the bands open and close – easy to check out with this setup.

The vertical dipole is fed at the center by a 1:1 balun and is further isolated with a 14 turn coil of RG-8 coaxial cable wrapped around a 7 inch diameter fiberglass form. The coaxial cable is routed at 90 degrees away for the antenna for about 20 feet before it runs across the roof of the house and then finally drops down to a 10 foot long ground stake. Here the coaxial cable passes though an Alpha-Delta 2 KW surge protector with the braid is connected to ground. Next, a 50 foot length of coaxial cable runs up the second floor shack.

The Dipole in Horizontal position

This antenna has been found to load great with SWR values of under 2.5:1 over the entire span of the bands that it covers.
A limitation of this antenna was the lack of coverage on the 75/80 and 160 meter bands. This deficiency was quickly and easily remedied by the addition of a Pixel Technologies Magnetic Loop Model PRO-1B all band “listening only” loop antenna.

I have recently made some back of the envelope calculations estimating the performance of this antenna.

The takeoff angle for this antenna should be around 15 degrees for 10 meters and between 25 and 30 degrees at 40 meters. The radiation efficiency should be equal to or slightly better than the typical 1/4 wave vertical antenna with 4 to 8 radials.

Note that due to the 1.25 inch diameter of the aluminum tubing that makes up the elements, the length of each segment of this vertical is about 5% shorter than it would be for a 12 gauge wire antenna. Due to the tuning traps for each of the bands, the physical length of this dipole, compared to a full length dipole, is 99% when it is used at 10 meters, 94% for 15 meters, 83% for 20 meters and 65% for 40 meters.

Taking into account the losses that are associated with the 150 feet of coaxial feed line, the balun that feeds the antenna, the traps and the overall antenna length for each band (listed above), the loss of power for the total antenna system can be estimated. The total power loss is around 1.5 db on 10 meters, 2 db on 15 meters, 2.5 to 3 db on 20 meters and maybe as much as 4 to 5 db on 40 meters when compared to a full length vertical dipole with a short feed line and a core type current balancing choke. If your signal is 10db over S9 at the receiving location, these losses will hardly matter.

Choosing one of the trap free antenna designs that are now available, in place of the Hustler design, could minimize the losses on the 20 and 40 meter bands.

Finally, now that all of the antennas in the plan have been installed the listening results for this antenna can be compared with my other antennas.

So far, some preliminary comparisons have been made on the 17 meter band.

The antennas that I can now select include a hex beam, an inverted V off center feed Windom (a long wire at this frequency) with good omni-directional characteristics due to the inverted V construction), my vertical dipole to which I have added the 17 meter band and a two wavelength horizontal loop antenna. Each has its own advantages and disadvantages and the shoot out between them has been very interesting. The four antennas can be set up to use the four antenna selector switch buttons on the front of the Alpha amplifier which makes A-B-C-D testing very easy and a lot of fun!

The preliminary 17 meter observations are:

1. The hex beam is always the most quiet and has the strongest signal in both transmit and receive mode even though it is only 24 feet above the ground.

2. The loop has an equally quiet noise level but delivers signal strengths that are about 6 db less than the hex beam.

3. The OCF inverted V has a noise level that is 2 to 3 db higher than the hex beam and produces signal levels that are between equal and 4 db lower.

4. The vertical dipole usually has a 3 to 6 db higher noise level, compared to the hex beam, and produces signal levels that are typically 3 to 6 db less than the hex beam. It does much better on transmitting than it does on receiving due to the high noise level when it is used as the receiving antenna.

Article by W6SDO orignally available at http://www.w6sdo.com/VERTICAL.html