[TowerTalk] ANTENNA RESONANCE VS HEIGHT ABOVE GROUND - DATA POINTS?

[Michael Tope]

Larry,

Have you tried taking the T-match out of the model, splitting the driven 
element, then looking at how far the resonance (zero reactance at driven 
element center) shifts in frequency when you go from 15' to 190'??

It would be interesting to see how that number compares with your 200 
KHz rule of thumb.

BTW, if you'all can afford it might be worth renting a cherry picker - 
one just tall enough to get rid of the ground interaction. Sidemount the 
yagi high enough to get rid of the ground interaction and then tune from 
the platform on the cherry picker.

Another alternative would be to setup up a low tram so that you could 
swing the back end of the yagi in close to the tower using the tag lines 
(at some point just high enough to get rid of the ground interaction). 
The guy on the tower could lash the back end of the boom to the tower 
with a bungy cord for stability and then do all the tuning in one shot. 
There is a safety issue here of course as you would need 100% confidence 
in your tram system. I once saw a tram guy anchor pull out once and send 
a Telrex 3 element 40 to slam into the side of a 180' tower. Luckily 
nobody was on the tower when it happened (IIRC - the climber had just 
come down).

73, Mike W4EF..................


Larry - K7SV wrote:

>Hello again,
>
>Thanks to the several folks that responded.
>
>After reading my first posting and the responses, it's apparent that I need 
>to better explain the situation.
>
>The antenna is 6 elements on a 56 foot boom. The design is patterned after 
>some 5 el 15M 36ft boom yagis that we've successfully used from NR4M for 
>maybe 15 years. Just a different band and an additional element.
>
>The antennas use a T-match. We tune the antennas by adjusting the driven 
>element length and the tap between the shorting bars between the T-match and 
>the driven element.  We depend on the natural capacitance developed in the 
>T-match rather than using external capacitors. As a result, the length of 
>the physical length of the driven element gets pretty close to the length of 
>the reflector. There's really nothing in the modeling that helps with the 
>Tee-match dimensions.
>
>For the 15s we simply raised the antenna up and down on a tram line over and 
>over to get the match right at about 70 feet. The 20 is obviously much 
>larger so doing all that tramming would be a pain in the butt. We currently 
>have 10 feet of AB105 sitting in concrete. It has the top section of the 190 
>ft of 105 bolted to it. The mast and rotator are mounted in the tower 
>section. The yagi was built on saw horses and the plan is to raise it to the 
>top of the mast which will put it at maybe 20 feet. The intent is to tune it 
>somewhat low in frequency there to get it in the ball park and then do the 
>final tuning as we run it up and down a tram to about a hundred foot level.
>
>As I said our guess based on what we have experienced in the past (based on 
>what we can remember!) is we'll start 200khz low and see what happens. We're 
>really wondering if anyone has done anything like a study that would give us 
>a better starting point than the 200khz guess?
>
>73 de Lar K7SV 
>  
>