### what coax ? MFJ-259 connects to balun via double male..or in extreme
rare cases, a 1-2 ft jumper.
Done on each ele of the 40m yagi.
But 40m els removed from boom, and hauled up the tower..one at a time.
80m rotary dipole mounted almost to top of mast..same deal though, mfj to
balun..via a double male connector
Yes, an aim 4170 will work as you described.
Jim VE7RF
-----Original Message-----
From: Grant Saviers
Sent: Tuesday, May 31, 2016 8:03 AM
To: Jim Thomson ; towertalk@contesting.com
Subject: Re: [TowerTalk] Change in Frequency As Antenna Height Rises
Perhaps this debate is really a measurement problem ---
I think the important thing to remember is that a SWR bridge is probably
NOT going to measure resonance when attached to some random length of
coax hanging in the air to an antenna.
What is needed is an analyzer that can be calibrated with the
measurement plane at the far end of the coax. A VNWA, AIM4170, SARK110,
etc. can be calibrated with S/O/L on the end of the cable so the
measurement plane is at the antenna. Then when jX = 0 you have
resonance. However, even then real R might not be 50 ohms, not 1:1
SWR. Otherwise some complex Z probably transforms to 1:1 SWR on an SWR
bridge at some other frequency than Fres depending on the length of coax.
Grant KZ1W
On 5/31/2016 7:01 AM, Jim Thomson wrote:
Date: Tue, 31 May 2016 08:27:27 +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:
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
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
_______________________________________________
_______________________________________________
TowerTalk mailing list
TowerTalk@contesting.com
http://lists.contesting.com/mailman/listinfo/towertalk
|
