I've been working on a 2m setup for hiking. My goal is to have the best possible antenna that can be used while walking, and that won't get damaged if I walk under a low branch or some other forest obstacle.

The setup so far: Off the top of a backpack with a steel frame I mounted a Diamond SRH77CA whip. It is connected by a very short length of coax to the HT in a side pocket of the pack, and then I have a speaker/mic clipped to a shoulder strap:

backpack with Diamond whip antenna

So far, so good, and the whip has proved to be both rugged and stable in its mounting.

In order to further improve the antenna's performance, I added a "Tiger Tail" counterpoise, following the directions here. (There's more about the theory behind that here.) It's a roughly 19" wire, 14ga, hanging from the base of the antenna down the side of the frame pack. According to the theory, this "transforms the 1⁄4-λ whip into a full-size center-fed 1⁄2-λ dipole". Yay for theory.

The PDF with the directions notes, "The hardest part of using this Tail is getting the wire to hang straight," but with the backback frame right there, I anchored it down the side using small zip ties in a few spots, and it's running straight up and down.

Now, the question: is the steel frame of the pack possibly messing with the performance of the Tiger Tail? Does a counterpoise need to hang in free space to work properly? Or by providing a solid, well-defined counterpoise path with that wire, have I prevented the antenna from capacitively coupling to the steel in the frame?

Thank you for your help, antenna wizards...

  • Tiger tail, really? How about counterpoise, radial, or ground plane? – Glenn W9IQ Jul 7 '17 at 21:58
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    @GlennW9IQ Exactly. "Tiger Tail" is an odd name for a radial. Those directions do make a good point: a 19" vertical is superior to most short 'rubber duck' antennas. – Mike Waters Jul 7 '17 at 22:17
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    I learned the term "Tiger Tail" from other hams, not from CB people. And while counterpoise, radial, or ground plane would all be technically correct, a Tiger Tail is of a specific physical construction (a vertical 1/4-wave wire), so it seemed apt to use the more specific term for this scenario. But if you want to call it a plain old counterpoise I won't be offended. – Dr Marble Jul 9 '17 at 18:16

Is the steel frame of the pack possibly messing with the performance of the Tiger Tail?

The frame is definitely interacting strongly with the antenna. With the tiger tail zip-tied to the frame you have something like a twin-lead transmission line made from the tiger tail and the frame tube, so you'll be exciting the backpack frame just as much as the tiger tail.

Whether or not that's detrimental to performance is difficult to say. I'd say if you really want the best possible, set up a field strength meter and measure the radiated power from the antenna empirically. You might find the tiger tail doesn't improve performance significantly, since the frame is already a pretty good counterpoise.

There's not really a reasonable way to add a counterpoise to that setup that doesn't interact strongly with the backpack frame besides possibly mounting the whip over a ground plane such that the backpack is "behind" the ground plane. However, such a ground plane would be pretty cumbersome, and I'd estimate the performance improvement is negligible.

  • Thank you Phil! The steel frame makes such a good physical anchor, it's too bad it has to be reckoned with as far as radiation goes. I don't have a field strength meter, but I bet I can track one down... I'm all for harvesting empirical evidence when the theory gets too complicated. I figured that the frame would already be acting as some sort of counterpoise, w/o the tiger tail. But the tiger tail directions spec'd that the wire should be cut to 1/4-λ, so I figured there was some benefit to a properly tuned, resonant counterpoise. I just hope I didn't made things worse. – Dr Marble Jul 7 '17 at 19:24
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    @DrMarble "Too bad"? When I see that backpack frame, I don't see an obstacle. I see an opportunity. Just because it's called a "backpack" doesn't mean it isn't also an effective antenna. Regarding the field strength meter, it's not too hard to make one. You might even use a cheap RTL-SDR or similar. – Phil Frost - W8II Jul 7 '17 at 19:35
  • Well it does complicate matters, that's all. If the frame was not reactive, I'd know I had close to an ideal 2m dipole. But I'll get or build a field strength meter on your suggestion, and test it out in different variations. Who knows, maybe the best setup will be running a short lead from the antenna to the steel frame itself, to use the whole thing as a ground plane. – Dr Marble Jul 7 '17 at 20:39
  • @DrMarble Phil is right, but I also suggest that you try some simple tests using distant stations. Compare the signal just holding the HT in your hand vs. mounted on your backpack. You should also try the radial just hanging loose away from the frame. – Mike Waters Jul 7 '17 at 21:39

There is a good chance that in your original configuration, the coaxial shield acted as a sufficient counterpoise. This is essentially beneficial common mode current on the outside of the shield.

As Phil correctly points out, the coupling to the backpack frame throws doubt on any theoretical analysis.

You may in fact be more effective by deliberately using the frame as the counterpoise instead of an added counterpoise. Sorry, but "tiger tail" sounds so CBish.

To do relative field strength measurements, simply involve a friend and a receiver with an S meter or other RSSI indicator. You cannot make applicable measurements by yourself without more complex remote monitoring capabilities as the presence of your body in the backpack will also influence the performance of your antenna system.

  • Thanks for your input. The coax cable was only 6 inches long. Would that still provide enough counterpoise length? Or does it need to be at least a 1/4 wavelength for it to do its job? – Dr Marble Jul 8 '17 at 2:38
  • It works as a counterpoise but perhaps not an ideal one. But even this probably interacts with the backpack frame. – Glenn W9IQ Jul 8 '17 at 12:10

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