[TowerTalk] Tuning raised radial verticals

[Bob Shohet, KQ2M]

My understanding is the mutual coupling between the four 1/4 wavelength verticals tends to raise the resonant frequency by ~ 5%, (that is what I have read in numerous places), meaning that you want to start with by cutting the 1/4 verticals for approximately 5% lower in freq. (longer) than where you actually want it to resonate.   My actual experiences with building multiple 4-squares on 40 and 80 is that the resonant freq. tends to be raised by ~ 3 %, not 5%.  Furthermore, if you use pvc coated wire like I do, that has the effect or lowering the resonant freq. by an additional ~ 2%.

So when I build or rebuild a 4-square, I cut the 1/4 wave elements for about 5% below the “target” resonant freq. and then the array typically has minimum reflected power (resonance) within ~ 1% of where I actually want it to resonate.  I use 3 – 4 elevated radials per element.  I do not know whether or not these resonant freq. shifts are the same or different with elements near the ground and 30 – 60 ground mounted radials, or whether or not there is a difference in resonant freq. shift with using a 4-square on open ground without trees.  (I am in very dense forest).

I suspect that terrain, forest vs. open field, and ground-mounted vs. elevated radials, affect the amount of coupling between elements and the freq. “shift” that one must take into account when cutting their elements.  It also goes without saying that most elevated 4-squares do NOT use a “spider-web” of twine (box + “x”) to keep the adjacent and opposite element spacing exact – differences in element spacing will likely affect the resonant freq. shift as well as any differences in the lengths of the 75 ohm feedlines between the elements and phasing box.   

I suspect that often when a 4-square is “off” from where it is expected to resonant, it is because either the element lengths, spacing and feedline lengths are not exact – or there is something NON-equivalent in the coax (Velocity factors can and often differ in the same cable at different points within the same roll of coax! – which is why each feedline must be measured first for physical length and then tuned for electrical length).

73

Bob KQ2M

From: john at kk9a.com 
Sent: Thursday, August 31, 2017 9:36 PM
To: towertalk at contesting.com 
Subject: Re: [TowerTalk] Tuning raised radial verticals

What is the reason for making the verticals 4% long?  I am far from being a
4sq guru however it would seem that you should have a choke, especially with
an elevated feedpoint. It is easy to make one using a short piece of RG-302
and a few ferrites that will leave your phased feedline long enough to still
reach the control box.


John KK9A


To: Bryon Paul Veal NØAH <bryonveal at msn.com>, Towertalk Reflector
<towertalk at contesting.com>
Subject: Re: [TowerTalk] Tuning raised radial verticals
From: Rudy Bakalov via TowerTalk <towertalk at contesting.com>
Reply-to: Rudy Bakalov <r_bakalov at yahoo.com>
Date: Thu, 31 Aug 2017 21:23:25 +0000 (UTC)
List-post: <mailto:towertalk at contesting.com>
I am measuring the impedance at the very base of the vertical with a
calibrated 
RigExpert. According to DX Engineering the vertical should be connected
without 
any baluns, matching networks...nothing. Furthermore, they are saying that
the 
vertical should be resonant 4% of the target array frequency. That is, in my

case, if I want to tune the array for 3.550 each individual vertical should
be 
tuned for 3.413.
Rudy N2WQFrom: Bryon Paul Veal NØAH <bryonveal at msn.com>To: Rudy Bakalov 
<r_bakalov at yahoo.com>; Towertalk Reflector <towertalk at contesting.com>Sent: 
Thursday, August 31, 2017, 4:34:40 PM EDTSubject: Re: [TowerTalk] Tuning
raised 
radial verticals
your seeing changes in the FEEDPOINT impedance using a tuned raised radial, 
thus your SWR will move  if yoy mess with them.  Suggest you consider using 
UnUns at the feedpoints to match the coax impedence to that of the 
verticals.  Also, a Comtek hybred coupler.....

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