[TowerTalk] Change in Frequency As Antenna Height Rises

[Jim Thomson]

Date: Tue, 31 May 2016 08:27:27 +0000 (UTC)
From: Dan Maguire <djm2150 at yahoo.com>
To: <towertalk at contesting.com>
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises

Jim VE7RF wrote:
>>> There is NO way in hell that the 66 ft long dipole, made from 1 inch OD tubing, is gonna shift 210 khz upwards in freq, when the tubing dipole is elevated from 30 ft to 60 ft.

I sure don't see anything in the model that would tax the capabilities of NEC so I guess we'll have to agree to disagree on that.  However, in playing around a bit more I did stumble on something that may be of interest to those folks who still put at least a little faith in antenna modeling.

I started with model 340-48H.YW from the ARRL Antenna Book CD, a 3L 40m Yagi with a 48 ft boom, and used the Scale.exe DOS program to convert the elements to monotaper.  I also changed the units from inches to feet.  Here's the result:

http://s33.postimg.org/hhm5hxebj/resonate4.png

Then I calculated the frequency of resonance (jX=0 at feedpoint, not necessarily minimum SWR), first for the 3L Yagi and then just for the stand-alone driven element (a dipole of length ~64.8 ft and diameter ~1.5 inches), at heights above Average ground from 2.5 to 100 ft.

http://s33.postimg.org/4yn6idnsv/resonate5.png

>From a height of 30 ft to 60 ft the Fres of the *Yagi* changed from 7.278 to 7.258 MHz (-20 KHz).  Over the same range the Fres of the *Dipole* changed from 7.132 to 7.349 MHz (+217 KHz).  Ground reflection works in mysterious ways when there is also mutual coupling between multiple elements.

Bob K6UJ wrote:
>>> With my 2EL Force 12 40M yagi, I followed Tom Schiller's guidelines and set it up initially at 15 feet above the ground. When I raised it up to 70 feet the resonance frequency increased by 20khz.

This is a 3L Yagi, not a 2L, but the model shows similar results.  From a height of 15 ft to 70 ft Fres changed from 7.244 to 7.255 MHz (+11 KHz).  Since a *Yagi* model seems to be in the same ballpark compared to actual measurements I don't see why a *Dipole* would be any different.

Dan, AC6LA

##  heres the problem.   f12  sez to check the resonance of each ele, individually.    IE: each of the 3 els  has a center insulator.     The REF  +  DIR normally have a shorting strap across their center insulators.   The DE normally has a helical hairpin
+ balun across it.   When tweaking say the REF for resonance  at 6900 khz, the shorting strap on the DIR is removed.   The helical hairpin + balun is also removed from the DE.  Each ele is tweaked one at a time to whatever freq  the manual sez to use.
Then the shorting straps re-installed  on  REF +  DIR.   Hairpin  + balun re-installed on DE.   A temp balun +  MFJ used to tweak each ele individually.   The F12  N style 40m eles are not full size.  They range from 59 ft for the REF...down 
to aprx  52 ft for the DIR. 

##  If the 40m  dipole alone shifts  upwards 217 khz.... then how come it doesnt shift  217 khz... when a motorized crank up  is  changed from 30 ft to 60 ft ??   It shifts very little.  
A shortened  80m rotary dipole shifts very little from 40 ft to 100 ft. 

##  I say something is amiss  in the model.    Anybody who has a 40m  full sized  wire dipole strung between trees..at  30 ft.... then decides to raise it to 60 ft a year later.... never saw a 217 khz increase in resonant freq. 

Jim   VE7RF