[TowerTalk] Modeling antenna traps

[Timothy Holmes]

Hi Don.

I already have the traps ftom an old tribander, and I am trying to use them
and the element tubing to make a pair of verticals.  Long term goal if they
work out is to make them into a phased array (2 element)

I would like to model them so that I can properly adjust them to see if it
is even worth my time, and what the performance will roughly look like

Thanks
Tim
W8TAH

On Sun, Jun 5, 2016, 1:41 PM Don <k9muf at comcast.net> wrote:

> 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 get
> the length and diameter measurements, but I dont know how to measure or
> model the traps. I am using MMANA, but I also have a demo of Eznec any
> input would helpful.
> Thanks
> Tim
> W8TAH
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