[TowerTalk] Fwd: Insulated elements, Yes - No?

[k9muf at comcast.net]

----- Original Message -----

From: "K7LXC--- via TowerTalk" <towertalk at contesting.com> 
To: pulsarxp at embarqmail.com, towertalk at contesting.com 
Sent: Wednesday, August 6, 2014 8:06:44 AM 
Subject: Re: [TowerTalk] Fwd: Insulated elements, Yes - No? 

> This brings up a big question in my small mind. Assuming you need to 
correct element length, I can see you can use SWR to adjust the driven 
element. But how do you correctly know how to correct or adjust the reflector, 
or for that matter any parasitic element? During the Dark Ages when I 
first got licensed, you just made the reflector around 10% longer then the 
driven element. How do you do it in 2014? (I really don't want to think of 
remote field strength measurements while adjusting the parasitic element.) 
If you correct the driven element by 1 inch to lower the SWR, do you then 
just adjust the reflector by the same 

What you do is to feed the element just like dipole and adjust it for 
the frequency of interest. This is easy to do with split elements like the 
F12. Just leave the other elements open and tune each one. 

I think F12 used something like 2.5-3% or so instead of the 10% you 
mentioned. If the manufacturer doesn't give you the element resonance, you 
could model it for your requirements. 

With non-split elements, I'm not sure how you'd do them. 

Cheers, 
Steve K7LXC 
TOWER TECH 
_______________________________________________ 

>>>>>>>You can measure the resonant frequency of non split elements with a "grid dip" oscillator. However this requires one to couple the GDO as loosely as possible and not rely on the reading of the GDO dial because they are not accurate enough. 

I have placed the element atop a 10' fiberglass step ladder removed a good distance from any other elements so as to prevent coupling to them and giving a false reading. I attached a short section of the boom with the same mounting arrangement that I intended to use for the yagi. I made a triangular shaped oscillator coil for the GDO to cover the frequency of interest and placed one leg of the triangle parallel to the element but removed from it just enough to barely give a dip. I then zero beat the oscillator with an accurate receiver to measure the resonant frequency. 

But before doing this, I modeled 12&17 meter yagis at the height that I intended to use with 1" dia. aluminum tubing just to make it simple. I then modeled each given element separately at a height of 10' to get their resonant frequencies at that height. I then adjusted each real element on the 10' step ladder to give the frequency that was modeled at 10'. 

I used this method to build a 3 element trapped WARC band beam with the driven element as a dipole on 30 meters. It seems to work OK. 

I also used the GDO to tune the traps, but that is a different story. 

Don, K9MUF 

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