HFU HF Underground
Technical Topics => Equipment => Topic started by: ChrisSmolinski on May 09, 2020, 2047 UTC
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I decided to work on optimizing my existing 500 ft Beverage antenna, rather than start work on a new vertical antenna. I took some bearing measurements of the existing antenna, and while not ideal, I don't think it is so far off from the idea 40 degree bearing as to not be useful. I may try to re-route at least a portion of the run, but today's efforts were spent on the input matching transformer and termination resistor. I built the antenna quite a few years ago, and never properly adjusted those two things, I just used the "try to get the best signal you can" method. Time to fix that.
My Beverage antennas has a coax cable run from inside the shack, which goes down the side of the house to a grounding block on a dedicated ground rod. The coax then goes underground through conduit about 70 ft or so and comes up above ground again to an isolated matching transformer. The secondary is tied to a ground rod and the feedpoint of the Beverage antenna. The far end of the Beverage antenna is then (or should be, more about that later) terminated through a resistor to ground.
A beverage antenna has a theoretical pattern with a peak towards the far end, and a low elevation radiation angle. Here's some plots of mine, made with my MININEC Pro program: https://www.blackcatsystems.com/software/mininec-antenna-analysis-modeling-software.html
(https://i.imgur.com/jSC2x9A.png)
(https://i.imgur.com/HgEtrGG.png)
Ideally, signals coming into the far end of a Beverage travel down to the feedpoint and into the coax. Signals coming from the back (feedpoint end) travel down to the far end and are absorbed by the termination resistor. But, if there are impedance mismatches, they can get reflected back down to the feedpoint and to the coax. Likewise if there's a mismatch at the feed, signals can get reflected back rather than go on to your receiver.
I used my Rig Expert Zero antenna/impedance analyzer, more details on it in this post: https://www.hfunderground.com/board/index.php/topic,49903.0.html
I used it in TDR (Time Domain Reflectometer) mode, which helps identify impedance mismatches. This first plot is before I started:
(https://i.imgur.com/xAWeXvz.png)
The green trace is the impulse response, the blue trace the step response. Ideally you'd like the blue trace to be flat without bumps/steps/etc. That means the impedance is constant the entire path, through all of the coax cable(s), the antenna, and the termination resistor.
One note: The velocity factor of the coax cable and antenna wire are NOT the same. So the lengths reported on the X axis are not accurate.
Doesn't look good. Looks pretty bad in fact. Not sure what is up with the matching transformer, I need to check that later. And you can see the big transient at the end of the antenna.
So I changed it out for another 450 to 75 ohm matching transformer:
(https://i.imgur.com/v2HP5va.png)
You can clearly see the transients at the feed and the far end. I walked down to the far and sure enough, some critter had chewed the wire. So that was going to need to be fixed.
The first step was finding the ideal turns ratio for the matching transformer. I won't go through all the steps I tried, but I ended up with an 8:3 turns ratio, or about 533 ohms to 75 ohms:
(https://i.imgur.com/K0AFQ9u.png)
Pretty close. Next step was the termination resistor. I decided to try 500 ohms as that is pretty close to the input impedance:
(https://i.imgur.com/nqhZ8h0.png)
Also pretty close. I think it could be tweaked, maybe to 550 ohms.
Next step, some real world testing. Hopefully conditions to Europe will be good tonight.
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Some more plots... SWR and impedance. Pretty flat once you get to the MW band.
SWR 0-10 MHz:
(https://i.imgur.com/7j2TxK8.png)
Impedance 0-10 MHz:
(https://i.imgur.com/tvmxcGR.png)
SWR 0-30 MHz:
(https://i.imgur.com/Ss3k2yj.png)
Impedance 0-30 MHz:
(https://i.imgur.com/I8R5Elj.png)
SWR Zoomed in on low end, 0-2 MHz:
(https://i.imgur.com/6eFKwXT.png)
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It'd be neato to see plots before and after rainstorms.
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Interesting measurement results.
Write what transformer you finally used, 8:3 unusual.
After Sky Loop and T2FD, I'd like to have Beverage. Unfortunately, the urban conditions do not give me a free safe space and direction to mount such a long antenna.
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Write what transformer you finally used, 8:3 unusual.
I used one of my Cyclops transformers, I just made a special unit with an 8:3 turns ratio. https://www.blackcatsystems.com/rf-products/cyclops_rf_ham_shortwave_radio_matching_transformer.html