Saturday, February 23, 2019

Experimental shunt fed tower

Since my antenna tower is grounded I searched for a way to shunt feed with a autotuner like a gammamatch. I found a article from N6TZ Hal on eham and although I read it I picked only the most important things out of it and used what I got. N6TZ is a broadcast engineer and his website is mainly about his antenna setup, a good read. Most important I think is that the beam on top is grounded to the mast and luckely it is, normally coax should also be grounded at the bottom but I have my RF systems isolator box so there is no galvanic connection between coax of the tower and the coax to the shack. Another important thing is that all wires and coax should be running inside the tower and in my case it is.

Autotuner box, note the
groundrod at the left.
A simplified description of the system: See the photo, running a wire (red lines) from top of the tower and bonded to the metal to the autotuner which is 6m (20ft) away from the tower. Running a flat copper ribbon strap a few cm underground from the "earth" of the tuner to the bottom of the tower and also bonded to the metal. The "earth" connection on the tuner is also connected to a copper rod into the ground, connected to the copper rod are 2x 10m long elevated radials. Coax from the autotuner is first going a few times through a ferrite #61 ring and together with the 12V supply going the the box at the bottom of the tower were I grounded the coax again. From the box it is connected to the coax going to my shack.

View from the autotuner to the mast.
Some people will think now that I made a grounded delta loop, but that is not the case. This system is called a top loaded shunt unipole vertical. However this is really a experimental antenna since you will never be able to predict or calculate any radiation pattern or angle on any band, it is just a matter of trail and error.

The shunt fed tower should work fine from 160m-40m. I tried to transmit on the system and it was tuned easily on all bands. I made some contacts on 60m and found I got unexpected surprisingly good reports. Switching from the inverted-V to the shunt fed tower didn't show me real differences.

I  was thinking about my testing method to compare antennas and thought the only way to get a real(time) comparisation result should be with 2 transmitters on WSPR transmitting the same power at the same moment. So I made 2 WSPR stations PE4BAS on the shunt fed tower and PE4BAS/P on the inverted-V both transmitting 1W on different frequencies in the WSPR spectrum at the same time. I'm shure this is the best method determing which antenna is best under same circumstances taking a fast changing propagation in mind. I tried to find stations around Europe and DX to compare. Only on 60m there were no DX stations on the WSPR frequency.


Hopefully this will reduce some RFI on both TX as RX.
Note that during my test transmission on 160m I had only 2 stations that received both PE4BAS on the vertical and PE4BAS/P on the inverted-V. Actually only those 2 stations received my 1W signal on the inverted-V. At the same time 1W on the vertical (shunt fed tower) gave me 50 (!) spots in one 2 minute transmission. Just to show you the extreme difference. 15 minutes later I worked Japan on 160m FT8.

Improvement of this system

Oh yes, reading the N6TZ's website a lot of things came to my mind. I could attach a tracer wire from top to bottom wrapped a few times around the tower to make it a more constant conducting element. Then the groundsystem. I only use 2 short elevated radials now, but everyone knows you need much more to increase the radiation. On both tower as autotuner should be much more conductors. I'm even thinking about connecting the fence wire, used for the beverage experiments, to the ground. For receive I'm already thinking about another antenna since the vertical doesn't receive as well or the same as my inverted-V.

Test setup 2 WSPR stations PE4BAS (tower) & PE4BAS/P (inv-V)

1 comment:

  1. Interesting, Bas. Your results prove the worthiness of this antenna on low bands and it is surprising that so many people seem unaware of it.

    My first time in hearing of this antenna was from Fred KT5X. With a very similar configuration he worked all US states in one night while QRP on 160 meters; he also obtained his QRP WAC on 160 meters. I emailed him, asking for the specifics and posted his response on my old blog back in 2010 and I've now re-posted it.

    Good luck with your new lowband pursuits!

    73 - John


Thanks for your comment. Bedankt voor je reactie. 73, Bas