RaDAR – Experimental antennas

ZS6BNE Open Wire Fed RaDAR Antenna

An antenna for the not so perfect world

By Eddie Leighton ZS6BNE

The idea of using a random length open wire fed antenna is as old as amateur radio itself. “Ladder line” can be easily home brewed.

In the past, I did not have much success using G5RV’s and the ZS6BKW improved antenna maybe because I did not try hard enough! However, reading up on the Net on random length, open wire fed, multiband antennas, my interest was once again awakened, especially for RaDAR, where, in most cases, the antenna system and the operating environment are far from ideal.

Having always used a QRP transciever for HF RaDAR communications, namely the FT-817ND, and an Inverted Vee I found myself tuning the length of an 80 meter Inverted Vee during the recent RaDAR contest because I had cut the lengths for the CW portion of the band and the only activity on 80 meters at the time was on the SSB portion of the band, the very last 100 kHz! That meant cutting a reasonable length off each side for resonance that the 817 could give full power (5 Watts) output. Note the SWR indicator on the 817 was a great help here!

After that experience, it was time to invest in an ATU. The most ideal option I found was the LDG z817 Automatic ATU (QRP) and I have never looked back. Not only did it help to work into an Inverted Vee cut SLIGHTLY off frequency but it opened a world of other options which is what I’d like to share with you here.

The open wire feedline can look somewhat bulky and as you may have read, such a line should not come into close proximity of metal objects. For quite some time now, I’ve been using an “Eskom” telescopic pole as an antenna mast in the field. Nothing could be more of an insulator than such an item. Indeed if it fails that test (High voltage) then it is discarded which is why some become available. If you’re very lucky you can get one at a very good price. I was extremely lucky, Henry ZS1AAZ, gave his pole to me!

The mast was the basis for my idea. I didn’t like the “Bulky” open wire feedline, especially for RaDAR. Anything that is carried unnecessarily is a waste of backpack space!

All transceivers have unbalanced coax connectors and having the incompatability with open wire feedlines. Fortunately there is such a thing as a Balun and I had one, a LDG 4:1 Balun. This was the key, opening the doors to both worlds!

Antennas are really an exact science. We talk about impedance, resonance and the theory behind antennas can be quite frightening. We pay dearly for optimum antenna designs. RaDAR in the field puts one in a position far away from being exact. Effective communications using low power and using simple items at hand is the objective!

Then I had a simple idea which became a bigger idea.

I used cable ties to fix my 4:1 Balun to the fiberglass mast, securing two flex wires to the sides of the mast. The mast then became the open wire spacer. Initially the wire lengths were quite random. I estimated the mast, when extended, to be over 6 meters in height and added another 20 meters of wire for each leg of the dipole / Inverted Vee antenna. So in effect, each wire was about 27 meters in length (I added another meter of wire, for taste). Note, nothing is exactly cut here, except for the two wires being the same length (Balanced)).

Using my FT817 and z817 ATU, I found the antenna could be tuned quite easily, at the push of a button!  No longer do I need to worry about 1/2 wave resonant lengths and coax losses to the feedpoint. Although Inverted Vee’s always worked for me, this arrangement is a winner! Open wire fed, random length flat top works in Inverted Vee configuration too. Tested using 5W (FT-817ND/z817 ATU) yesterday. Comms good locally and into Natal on 40m. The antenna tunes on all bands 80 to 6m including 30m and WARC bands!

John Green commented, “Eddie neat tuned feeder multiband dipole, clever idea to use the fibreglass pole as the spacer for the ladderline at the bottom. I have just received some packets of ladderline clips, still to try them out”.

Vidi la Grange commented, “Eddie, I love this idea and also use a telescopic pole like yours. I added a small (hard to find) pulley to the top and also an aluminium plate to which I can clip 3 stay ropes”.

Indeed, on my “To do” list, I had to find a better way of “Joining” the feedline and the antenna main and came up with this idea.

RaDAR_Antennas

My comments on Facebook were, “29 January 2011 – My new RaDAR Multiband HF antenna. The open wire feeder tensioner system. Note, the “Pulleys” are made from rubber tap washers, the axle, a 6mm bolt and nylock nut. The flex wire used for the feeder / antenna, rides on these”.

Allen Wood commented, “Getting it spot on here Eddie. I have basically something similar but not happy with my feed line yet. The idea is to make the antenna as versatile as possible to suit any operating location and operating conditions”.

The secret to the open wire feedline tensioner system, was the use of Bungi cords. The following picture shows how this was done while the antenna was in a Delta loop configuration. The pulleys used were from an ineffective experiment done before the final “Tap washer” pulley idea! The metal pulleys worked much better here!

RaDAR_AntennasDeltaLoop

Delta Loop / Open wire feed, tensioner.

It was time to go into the field for antenna tests, the results were quite satisfactory! It turned out that, with one antenna, I could change to a different configuration quite easily, and quickly! The following were documented during the tests.

Antenna A – Multiband, open wire fed, doublet (Inverted Vee configuration).

Bands : 80m 40m 30m 20m 17m 15m 12m 10m

Open wire feedline length : +- 6 meters

Inverted Vee leg lengths : +- 21 meters , each

Antenna B – Multiband, inverted L (One leg of the doublet).

Two 20 meter lengths of flex wire were used as radials and connected to the earth terminal of the 4:1 balun.

Bands : 80m 40m 30m 20m 17m 15m 12m 10m

Inverted L leg length : 21 meters + 6 meters = +- 27 meters.

Antenna C – Multiband, open wire fed (Top), (NVIS) Delta Loop.

Bands : 40m 30m 20m 17m 15m 12m 10m

Open wire feedline length : +- 6 meters

Loop length : 21 + 21 meters = +- 42 meters. Doublet legs folded back to the base and joined.

Using an antenna system like this, gives me greater peace of mind, knowing under any circumstances, I will be able to communicate from the field.

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