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I seem to have forgotten some things I learned years ago for my amateur radio exams. If I am building a 2 band shortened dipole and want a trap between 80 and 160 do I make the trap resonant on 80 or 160 or elsewhere? I guess I may be mistakenly viewing the trap as a gate that allows certain frequencies and not others. Is this correct thinking (albeit simplified)? Can anyone tell me what I should be shooting for here or help me understand the function of traps in a shortened dipole and how I would go about making a trap for a 80/160 dipole? Full length 80 is fine, with a short 160, would be best.

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Traps are L-C cicuits that become high impedance at their resonant frequency. It's common to make traps by winding lengths of coax around circular forms like PVC pipe of various diameters. The center conductor of one end is soldered to the shield of the other end, and the remaining center conductor and shield connections are connected to the antenna elements.

The series-connected inner conductor and shield of the coiled coaxial-cable act like a bifilar or parallel-turns winding, forming the trap inductor, while the same inner conductor and shield, separated by the coaxial-cable dielectric, serve as the trap capacitor.

The resultant parallel-resonant LC circuit exhibits a high impedance at the resonant frequency of the trap and effectively disconnects everything after the trap from the antenna. Any inner traps (which are operating below their resonant frequency) function as loading coils and shorten the overall physical length of the antenna.

In your case you'll want at least 3 traps. One will be resonant at the lower end of the 80m band. This will disconnect the 160m portion of the antenna when you are transmitting on 80m. You'll want one between there and the feedpoint with a resonant frequency above the highest frequency you'll use on this antenna. This one is really a loading coil to make up for the physcilally short antenna length. The other will be between the 80m trap and the end of the entenna. This one should be resonant below the 160m band and will serve as a loading coil on 160m.

A couple of notes are in order:

  • Coaxial-cable traps have a relatively high Q, which results in a relatively sharp frequency resonance.
  • Which means traps need to be tuned for their resonant frequency, you can't use a cookbook approach here, if they are not right on, you've just made a dummy load. To tune the traps, you'll need a dip meter or a sufficiently featured antenna analyzer
  • Top band (160m) is a harsh mistress. It rewards the dilligent worshiper who builds full size antennas and punishes all others. You might consider a folded dipole if you have room for a full sized 80m dipole.

A Further Edit: A folded dipole is a dipole antenna with the ends folded back around and connected to each other, forming a loop as shown below:

enter image description here

The folded dipole antenna is resonant and radiates well at odd integer multiples of a half-wavelength (0.5λ, 1.5λ, etc.), when the antenna is fed in the center like a regular dipole. It can be made resonant at even multiples of a half-wavelength ( 1.0λ, 2.0λ, etc.) by offsetting the feed of the folded dipole (closer to the top or bottom edge of the folded dipole in the picture).

So you could make an off-center fed folding 80m dipole that will also be resonant on 160m. The feed point impedance will be higher than for a typical dipole, but that's usually not a problem.

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  • $\begingroup$ Hi AB3RY, Thanks for that answer, it's very useful. I don't need the whole antenna shortened, just the 160m portion of it. Could I get by with a single trap or am I going to be forced to use 3? $\endgroup$ – Marcus Nov 21 '13 at 19:51
  • $\begingroup$ Just saw your edit - it was my understanding that a folded dipole is still full length, just doubled over to increase bandwidth. Am I incorrect? I have a dip meter and antenna analyzer, no problem there. I barely have room for a 80m dipole with the legs at around 130 deg. I don't know about 160, that might be tough. $\endgroup$ – Marcus Nov 21 '13 at 20:01
  • $\begingroup$ I've updated my answer to clarify what I meant with the folded dipole. I hope that helps. Bent leg dipoles like you are describing do work rather well. In theory they will favor the interior angle for propagation, but in practice there are enough other factors that this is negligable. I have an 80m inverted V dipole at about 50' and an 120-130 degree included angle and it performed great last weekend for SSB Sweeps. $\endgroup$ – WPrecht Nov 21 '13 at 20:15
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    $\begingroup$ A resonant folded dipole is not half the size as an ordinary dipole -- it is the same size, with twice the wire. That is, in the figure you have, $L=0.5\lambda$, total wire length $=1\lambda$. Also, the trick you mention of offsetting the feedpoint to get resonance at even harmonics works with ordinary dipoles also. $\endgroup$ – Phil Frost - W8II Nov 21 '13 at 22:45
  • $\begingroup$ Also, the higher impedance is a problem. It's a lot higher, like four times higher. You will need a 4:1 balun to get a reasonable match to $50\Omega$ coax. $\endgroup$ – Phil Frost - W8II Nov 21 '13 at 22:48
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There are many kinds of traps, but the most common kind is a parallel LC circuit:

enter image description here
Source: article from w8ji.com

At resonance, this circuit presents a high impedance. At all other frequencies, it presents a low impedance. So, an 80/160m trapped dipole will be an 80m dipole, with traps at the end of that, with additional wire to resonate it at 160m. To 80m, the traps look like high impedances, effectively disconnecting the extra wire for 160m.

The traps themselves will shorten the amount of wire necessary to achieve resonance at 160m, similarly to a loading coil. However, they don't shorten the 80m section of the antenna. If you barely have enough room for an 80m dipole as it is, you need some additional shortening action.

Easiest is probably to bend the antenna to make it fit. You can stick a 90 degree bend in the antenna and it will still work. The tuning will be somewhat changed, but not radically. You could also add additional loading coils or hats, though these are somewhat harder to construct.

Another option, if an 80m dipole is already very close to your maximum size, is to put up a simple 80m dipole, forget the traps, and instead have a tuning network near the feedpoint that you switch in (or isolate with filters) for 160m operation. The advantage to a trapped antenna is the additional length on the longer bands maintains higher efficiency there, but if your 160m sections are extremely shortened then you aren't gaining any efficiency, but you are making the antenna difficult to construct and tune. If you put loading coils in the 80m sections then you shorten the whole antenna, but compromise performance on 80m when you don't need to. If you have a low-loss feedline, you can even do this with a tuner in the shack, especially considering how low feedline losses can be at 160m.

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  • $\begingroup$ Thanks for the answer phil. I think I have some wiggle room on the dipole so I've built 2 traps resonant on 3.8 (shot for 3.6 but I'll take 3.8) that I'll put at the end of the 80m legs and then add wire as necessary for 160. I have an antenna that covers the rest of the bands, I just had nothing for the low bands and the 160m CW contest is coming up, I didn't want to miss it. I appreciate the answer. $\endgroup$ – Marcus Nov 22 '13 at 3:28
  • $\begingroup$ Phil - quick question. If have built 2 traps resonant at 80m to put at the ends of the 80m dipole, is there any way to calculate how much shorter the 160m additions will be? $\endgroup$ – Marcus Nov 22 '13 at 15:10
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    $\begingroup$ @Marcus There is, but to get an accurate result, you will need to accurately measure the reactance of your trap. I'm guessing if you had the equipment to do that, you wouldn't be asking. In practice, it's probably easier and more accurate to start with a full length 160m dipole, then incrementally shorten it until it's tuned. This is easier, and also takes into account the antenna's surroundings (which can be significant), and doesn't require anything more than an SWR meter and a transmitter. $\endgroup$ – Phil Frost - W8II Nov 22 '13 at 15:58
  • $\begingroup$ Thanks Phil. After looking at buying all the wire and whatnot to make this happen I think I'm going to end up buying a DX-LB from Alpha Delta. I don't mind building my own antennas and have built many but this lot only has room for about 130 ft so there's no way, even folded, I'm going to get a 160m dipole in the lot. I did some checking with the figures on these traps and it looks like I could get the dipole down to 220ft but it's not enough. The DX LB is pretty short, with short attendant bandwidth but all antennas a compromises so I'll make do. Appreciate your time, I've learned a lot. $\endgroup$ – Marcus Nov 22 '13 at 19:19

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