Saturday, February 25, 2012



My experimentation with backyard/stealth antennas has resulted in this succesful multi-band Windom inspired design. 

A Windom is basically a horizontal long wire fed by a single wire feeder (offset T) tuned against ground or a counterpoise. 

I started with the horizontal section cut to resonate as a half wave. A GDO can be used and at 3.500mhz (80m cw) I discovered that 135ft (as predicted by calculation) was exactly right.

This aerial is very modest in design, height and cost. However it is a good performer and works well on all bands.

I then created a 30ft vertical single wire feeder (supported by the fibre pole)  and experimented with the point at which the vertical meets the horizontal. The characteristic impedance of the feeder (around 600 ohms) is matched to the impedance at one particular horizontal tapping point. The single wire feeder then becomes a transmission line and the length of the vertical section becomes immaterial. Under these conditions the feeder will not radiate much and the antenna becomes a horizontal 1/2 wave. This balanced state of affairs can only be achieved on one band. 

I discovered that by modifying the windom I could build a multi-band antenna which works better than a traditional vertical groundplane. 

By connecting the vertical feeder at 30% from one end (as described in many texts) of the horizontal wire I ended up with a really super efficient top loaded vertical antenna that works on all bands (with an ATU) and does not require loading coils, baluns, traps or extensive radials. I formed the vertical section using a 35ft fibreglass roach pole which serves to help support the horizontal wire and counteract the tendancy for a long wire to sag in the centre. (ala Inverted V). 

This is a great way of creating a vertical far better than a ground plane. The current portion of the standing wave is at a high elevation and radials are less important. This design is often called an upside down vertical. This is a killer antenna on all bands. 

I employ a MFJ993 auto tuner working against ground and located remotely with a well grounded 50ohm coax feed to the rig. This arrangement eliminates RF in the operating shack. 

I have been surprised at the low initial SWR on the ham bands. On 160m I have 1:1 swr and a low angle of radiation with no tuner needed. On 80 and 40 the radiation is a mix between horiz and vert, providing an all round response. On the high bands the radiation pattern is frankly complex but still useful. The MFJ993 tuner is essential to pull in the SWR over all bands. With this design I have not found any untunable frequencies. 

A significant improvement was obtained by installing a 50 foot long counterpoise wire. (3 feet above ground). I have modelled the antenna design in Eznec and I find test results very close to what is predicted. A counterpoise is really essential then extensive radials are not required. 

I have no idea how to measure radiation resistance, but I am convinced this antenna has a much higher feed point RR (I estimate 100ohms) than a ground plane (35 ohms) so providing significant efficiency gain over a ground plane antenna with radials. 

Performance on 40m is outstanding in most situations cutting through dx pileups without trouble. Florida USA on 160m, VK on 80, W6 and VK on 40. etc. All bands work well, I believe better than can be expected for such a modest design and low outlay from a very urban location. 

More efficient than a trap vertiacl. (at least 2x better)
Cheap to home brew.
Covers all bands easily with an ATU
Low angle radiation 
Low visual impact.

Little directivity with complex patterns on the higher bands
Sensitive to local noise sources.
Can wave about in the wind so needs constant maintenance.

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