[CQ-Contest] [TowerTalk] Modeling antenna traps

Don k9muf at comcast.net
Sun Jun 5 13:41:32 EDT 2016

Hi Tim,

I haven't seen any replies to your question so perhaps I can give you a few clues.
I can't help you with modeling the traps, but you can do it by experimenting.
First of all you should determine what frequency you want the traps to be resonant at. 
It might seem that they should be resonant in the higher frequency band that you want to
separate from the lower band but that can cause high losses. This was discussed in an article in Ham Radio magazine of August, 1987 by Yardley Beers, W0JF and also by w8ji. For the lowest loss, the trap should be resonant at a frequency about midway between the bands.
I have build a trapped Yagi beam by measuring the trap frequencies with a grid dip oscillator.
This can be a little tricky and requires some practice if you haven't used a GDO before. When measuring the trap, it should be disconnected from other elements of the antenna and should be separated from other metal objects. There may be other ways to measure the frequency, but I haven't tried them. You can also calculate the inductance and capacitance required to the resonate the trap, but then you have to have a way to measure those values. If you are using old trap assemblies, you are pretty much limited by the capacitance built into the trap, but you can adjust the inductance by changing the number of turns in the coil. Once you have the trap made, you should make arrangements to adjust the lengths between the trap and the antenna elements.You can then adjust the frequency of each band by using an SWR meter or an antenna analyzer starting at the highest frequency band first.

Good luck.

Don,  K9MUF

----- Original Message -----From: Timothy Holmes <taholmes160 at gmail.com>To: Tower Talk <towertalk at contesting.com>, cq-contest at contesting.com <CQ-contest at contesting.com>Sent: Wed, 01 Jun 2016 20:52:27 -0000 (UTC)Subject: [TowerTalk] Modeling antenna traps

Hi folks ...

Bit of an off topic here.

I want to model a vertical made from old tribander parts. Its easy to getthe length and diameter measurements, but I dont know how to measure ormodel the traps. I am using MMANA, but I also have a demo of Eznec anyinput would helpful.


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