Towertalk
[Top] [All Lists]

[TowerTalk] Change in Frequency As Antenna Height Rises

To: <towertalk@contesting.com>
Subject: [TowerTalk] Change in Frequency As Antenna Height Rises
From: "Jim Thomson" <jim.thom@telus.net>
Date: Wed, 1 Jun 2016 07:24:18 -0700
List-post: <towertalk@contesting.com">mailto:towertalk@contesting.com>
Date: Wed, 1 Jun 2016 08:30:05 +0000 (UTC)
From: Dan Maguire <djm2150@yahoo.com>
To: <towertalk@contesting.com>
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises

Jim VE7RF wrote:
>>> 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.

Well, now we're making some progress at reaching common ground.  I created a 
dipole model with 1.5" diameter aluminum wires and adjusted the length for 
precise resonance at 7.15 MHz in free space.  Then I changed the length to 85% 
of the self-resonant length, added loading coils mid-way out each leg, and 
adjusted the coil inductance to again get precise resonance at 7.15 MHz.  And 
finally I changed the length to 70% of the self-resonant length and again 
adjusted the coil inductance to get precise resonance at 7.15 MHz.  Coil Q was 
kept constant at 200.

Then I switched from free space to average ground and varied the height of each 
dipole from 10 ft to 100 ft, this time adjusting the *frequency* for precise 
resonance at each height while keeping the length and coil inductance (if any) 
constant.

http://s33.postimg.org/v51yi2n5b/resonate6.png

100% length (69.79 ft), no loading, +218 KHz change from 30 ft to 60 ft.
85% length (55.93 ft), 3.297 uH coils, +137 KHz change from 30 ft to 60 ft.
70% length (46.06 ft), 6.721 uH coils, +78 KHz change from 30 ft to 60 ft.

So it seems that more loading results in less change in resonant frequency as 
the height above ground is varied.

Dan, AC6LA

##  each f12 40m N style ele starts at  1.875 inch OD....and tapers down to 
just .375 inch OD at the tips.   The oem F12 linear loading uses  3/16 solid al 
rods...and starts at 7 ft 9 inches out from the boom. 
Both LL rods extend outwards towards the tips.  ( as noted in a previous post, 
I replaced the LL rods with .375 inch silver plated cu tubing coils). 

##  If u see a  78 / 137 khz change  from 30 to 60 ft....then every rotary 
dipole maker  would be warning its end users to tweak tips  vs installed 
height....and they dont.   And I sure didnt see any 218 khz change   when
increasing the height of a full sized  wire  dipole on 40m.    ( 40 ft to 76 
ft).   It barely shifted.   So what gives ?   Is the model wrong ?   Something 
is wrong somewhere.  The software doesnt  jive with real world results. 

Jim   VE7RF



_______________________________________________



_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk

<Prev in Thread] Current Thread [Next in Thread>