Will this compromised antenna work?

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Brian - K3ES
Posts: 103
Joined: Fri May 27, 2022 12:55 pm
Location: NW Pennsylvania

Will this compromised antenna work?

Post by Brian - K3ES »

During a cross-country driving trip, we have been stopping to do Parks on the Air (POTA) activations at a number of state and national parks. At some locations it has seemed best to me to keep a very low profile, so that I do not distract others who are enjoying the area. For optimum stealth, I choose to use my Elecraft KX2 with its companion AX1 antenna. The setup shown in the next picture comes from an activation in a parking lot at the Mount Rushmore National Monument. I found I could clamp the AX1 to the base of the VHF/UHF mobile antenna on the fender of my truck, run the counterpoise wire over the hood and toss the remainder directly under the truck, and run the feedline in through the driver's side window. I was uncertain about the effectiveness of this setup for three reasons:
  • The AX1 (a base-loaded 4 ft whip) starts as a compromised antenna system
  • Having the whip parallel to and separated by only 2 inches from the mobile whip could have unpredictable effects
  • Running the counterpoise wire mostly under the truck would also have uncertain effects
Mt Rushmore QTH
Mt Rushmore QTH
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What would you expect from this antenna setup?

The next picture shows the QSO map from the activation. It took less than 30 minutes to log 11 contacts.
Mt Rushmore QSO Map
Mt Rushmore QSO Map
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Based on this success, I decided to use the same setup during an activation in Yellowstone National Park. All went well for the first contact, then a large recreational vehicle (RV) pulled in to the next parking space. You can see it in the next picture. The RV was longer than my truck, and despite the perspective seen in the picture, it was actually higher than the top of the AX1 antenna.
Old Faithful QTH
Old Faithful QTH
11846.jpeg (243.25 KiB) Viewed 2478 times
What would you expect from this antenna setup?

The next picture shows the QSO map from the activation. It took 40 minutes after the first unobstructed contact to get 10 more contacts for a total of 11. This was not as quick, but persistence paid off.
Old Faithful QSO Map
Old Faithful QSO Map
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I will say that I was surprised at my success with this configuration. A couple more pieces of might be useful. First, all contacts were made on the 20m band running CW mode at 5 watts. Second, solar conditions were good for both activations. I am definitely interested in your thoughts.
Andrew (grayhat)
Posts: 175
Joined: Sat Jun 11, 2022 2:59 pm

Re: Will this compromised antenna work?

Post by Andrew (grayhat) »

what's the length of the other antenna, and which band you made the contacts on ?
Brian - K3ES
Posts: 103
Joined: Fri May 27, 2022 12:55 pm
Location: NW Pennsylvania

Re: Will this compromised antenna work?

Post by Brian - K3ES »

Hi Andrew,

Both antenna setups were identical, except for having the RV pull into the parking space beside my truck early in the second activation (a car towing a smaller travel trailer also pulled in on the other side, so i was truly boxed in). The mobile antenna is a dual band 2m/70cm whip. All operations at these activations were in the CW portion of the 20m band. Transceiver was Elecraft KX2, and its internal tuner was used for matching. The 13 ft counterpoise wire ran down over the front of the truck, with the remaining length being tossed under the truck onto an asphalt-paved surface.

Best 73 de Brian - K3ES
Andrew (grayhat)
Posts: 175
Joined: Sat Jun 11, 2022 2:59 pm

Re: Will this compromised antenna work?

Post by Andrew (grayhat) »

length of the V/U antenna ?
Brian - K3ES
Posts: 103
Joined: Fri May 27, 2022 12:55 pm
Location: NW Pennsylvania

Re: Will this compromised antenna work?

Post by Brian - K3ES »

Whip length of the VHF/UHF antenna is 32-3/16 inches or 82 cm.
Andrew (grayhat)
Posts: 175
Joined: Sat Jun 11, 2022 2:59 pm

Re: Will this compromised antenna work?

Post by Andrew (grayhat) »

Thank you, on monday will put together a NEC model, I'm curious about the pattern
KL7MJ
Posts: 97
Joined: Mon May 15, 2023 9:06 pm

Re: Will this compromised antenna work?

Post by KL7MJ »

Brian,
With two successful activations under your belt, I'd have to say that yes, your antenna works!

73,
Mike, KL7MJ
Brian - K3ES
Posts: 103
Joined: Fri May 27, 2022 12:55 pm
Location: NW Pennsylvania

Re: Will this compromised antenna work?

Post by Brian - K3ES »

Thanks Mike,

I was uncertain, but the AX1 continues to amaze me. A compromised antenna is, first and foremost an antenna.

Brian - K3ES
Andrew (grayhat)
Posts: 175
Joined: Sat Jun 11, 2022 2:59 pm

Re: Will this compromised antenna work?

Post by Andrew (grayhat) »

Brian

I've put together a "quick and dirty" model in 4NEC2, didn't include the car (which will have effects) but at a quick test, the antenna shows a multi-lobe radiation pattern with low takeoff angles and a preferred direction toward the shorter (VHF/UHF) whip, will try playing a bit more with the model, but in any case I hope that such preliminary infos may be useful
Andrew (grayhat)
Posts: 175
Joined: Sat Jun 11, 2022 2:59 pm

Re: Will this compromised antenna work?

Post by Andrew (grayhat) »

<GRRR> :( :( :(

I stand corrected, when putting together the model, I forgot a dot, so I was testing a frequency of 14000 instead of 14.000 MHz, go figure :(

Ok, this time I got it right, aside from the lack of "the car" (but I'm planning to add it !) here's what the antenna looks like when installed near the V/UHF whip

download/file.php?id=178
brian01.jpg
brian01.jpg (57.65 KiB) Viewed 1876 times
basically the shorter whip doesn't seem to affect the AX1 in a relevant way, gain isn't "stellar" and may probably be worse, but then and again, the model is still "very raw" so I'd need to improve/refine if willing to drill further down into "how it behaves" :D
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