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Author Topic: Choosing resonant frequency for coax antenna traps?  (Read 2189 times)
G7MRV
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« on: April 14, 2013, 08:09:26 AM »

OK, I understand the theory behind the operation, and the construction, of coax traps, and I understand the principle behind not having the trap resonant on the frequency of operation.

But, although everything ive read says to design the trap to be resonant a little way off frequency/just out of band, I cannot find anything that gives a suitable offset figure, so dont know just how far from my intended operating frequency I should design the trap for?

Take for example, the first trap I need - 10m. This antenna will be used for SSB work around 28.3 to 28.6MHz. How far away should I design the trap? Is there a percentage figure that will allow me to decide for any frequency/band? If we take my desired operating frequency as 28.45MHz, should I go 500kHz, or a meg, higher or lower?

Also, nothing hints at which way to go, up or down! I can see in principle that going lower than the band will perhaps shorten my antenna, and above the band lengthen it. Either is fine (perhaps a combination, to aid in better separation between the 10, 12 and 15m traps), but is one direction more preferred than the other? and if so for what reason?

Im about to start construction of the 10m trap, so really need to know how far from the operating frequency to design it for

Any advice gratefully received
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WB6BYU
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« Reply #1 on: April 14, 2013, 10:32:22 AM »

If you are designing an antenna for two bands, the trap frequency should be between
the two bands so that it appears capacitive on one band and inductive on the other.
(That assumes that you only have a single current maximum on each band, so
not using 3/4 wave elements.)

The further from resonance on the upper band you go, the more the length of the
outer wire affects the tuning of the higher band.  For example, for a 10m / 20m
dipole the traps could theoretically be tuned to 15m, but then you are juggling
both sets of wires to try to get the resonances centered on both bands, and this
also depends on the L/C ratio of the traps.

Meanwhile, because the wires beyond the traps contribute to the radiation when
the traps aren't resonant, the feedpoint impedance goes up somewhat.  This is
sometimes helpful for vertical antennas which have a low feedpoint impedance,
but will result in a higher SWR for dipoles that are going to be close to 75 ohms
to start with.

It gets even worse if you want more than two bands on the antenna, as you quickly
end up with too many variables to juggle.  Even working out a solution using EZNEC
or some other modeling program can take time.

So if you are building a multi-band antenna, the easiest approach is to make the traps
resonant in the desired band.  You can try tuning them just below the band edge
(such as 27.5 - 28.0) and see how that works - if you get too much interaction you
can then move them up slightly.
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W5WSS
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« Reply #2 on: April 14, 2013, 10:50:42 AM »

And lower is longer leaving adjustment if you want to do so by necessity.
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W5WSS
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« Reply #3 on: April 14, 2013, 10:51:36 AM »

allowing shortening adjustment provision
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G7MRV
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« Reply #4 on: April 14, 2013, 01:06:35 PM »

The design was for a 5 band trap vertical, the bands being 10, 12, 15, 17 and 20m. A bit of further advise and research strongly suggests against this in a single antenna due to difficulties with closely spaced bands. I still 'need' to cover the 5 bands with the antenna, but am leaning towards a 3 band and a 2 band with a common feedpoint and support. So I guess im looking for trap frequencies for a 3-band 10-15-20m design and a 2 band 12-17m design
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N4CR
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« Reply #5 on: April 14, 2013, 03:04:53 PM »

The design was for a 5 band trap vertical, the bands being 10, 12, 15, 17 and 20m. A bit of further advise and research strongly suggests against this in a single antenna due to difficulties with closely spaced bands. I still 'need' to cover the 5 bands with the antenna, but am leaning towards a 3 band and a 2 band with a common feedpoint and support. So I guess im looking for trap frequencies for a 3-band 10-15-20m design and a 2 band 12-17m design

You can see how to convert a Hustler 4BTV to cover 12 and 17 by adding vertical quarter wave elements spaced away from the middle element. This can work just as well on a wire antenna if you get the spreaders right. You'll probably only want 1 spreader at the top of the 17 and lengthen the 12 element with nylon to reach it.

http://www.hamuniverse.com/kl7jr4btv1217mod.html

This is a heck of of a lot easier than using 12 and 17 meter traps.
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73 de N4CR, Phil

We are Coulomb of Borg. Resistance is futile. Voltage, on the other hand, has potential.
G7MRV
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« Reply #6 on: April 15, 2013, 02:28:52 AM »

Yes, saw that whilst looking at commercial trap vertical designs.

The extra 17m section though isnt strictly a trap antenna, its a plain vertical in parallel. Essentially, making it a hybrid fan vertical. One element a straight vertical, the other a trap vertical.

This has actually led me back to an idea I was playing with a while ago - a 5 band fan vertical. The big difficulty is mechanical - It needs several spreader 'spiders' to hold the elements at a  suitable distance from the mast (one element can run up the mast). If I can solve that difficulty, the rest should be comparatively easy!
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VA3RR
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« Reply #7 on: April 16, 2013, 07:25:53 AM »

This has actually led me back to an idea I was playing with a while ago - a 5 band fan vertical. The big difficulty is mechanical - It needs several spreader 'spiders' to hold the elements at a  suitable distance from the mast (one element can run up the mast). If I can solve that difficulty, the rest should be comparatively easy!

You could try a G7FEK-style fan antenna:

http://g8jnj.webs.com/G7FEK%20antenna%20revisited.pdf
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G3TXQ
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« Reply #8 on: April 16, 2013, 11:09:16 AM »

I believe this issue of trap losses at resonance is over-stated.

I recently measured a 5MHz coax trap wound with RG58, and at resonance it had an equivalent parallel resistance of over 70,000 Ohms. Try plugging that value into an EZNEC trap dipole model and you'll find the losses are insignificant.

Steve G3TXQ
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