[TowerTalk] RG11 75ohm and 4 square 80m

[Paul Playford]

Very important when using elevated radials: they will affect the resonant 
frequency of the element.  When I installed my 4 squares I first installed 
two wires in the form of a 1/2 wave dipole (fed at the center) at their 
final elevation above ground.  Then trimmed them to length at my target 
frequency of the array.  Then I cut the rest of the radials to the same 
length and installed them.

I use 4 radials under each element but a friend of mine likes to use 8 
radials.  I suspect 8 radials will work a little better, but how much?

Do not let the radials touch each other where they cross.  On 40m I burned 
holes through two layers of 600 volt insulation where they used to touch.

After all of the radials were installed I then installed the vertical 
elements and tuned them per Comtek instructions against the radials.

Everyone refers to the feed lines as phasing lines, however I call them 
impedance transformers.  A bit of 1/4 wavelength transformer magic: 36 ohms 
at the feed point of the element becomes 72 ohms at the Comtek box - 
parallel two elements (the side elements) and you are back to 36 ohms.  So 
the front element has an swr of 1.5:1, the side elements have an swr of 
1.5:1 and the rear element has an swr of 1.5:1.  This assumes the output 
ports on the Comtek box are 50 ohms.  Neat.

Which means that 1/2 wavelength lines will not work, gotta be either 1/4 
wavelength or 3/4 wavelength.

You must use foam dielectric cable for the lines with a velocity factor of 
about .8 to physically reach from the Comtek box to each element.  Solid 
dielectric coax will be too short.

And don't believe the manufacturers spec when it comes to velocity factor: 
the first set of lines for my 40m 4sq were almost too short.  2nd set 
(different coax manufacturer) were plenty long.

My first set of impedance transformers (phasing lines) for my 80 and 40m 4 
squares were cut using an MFJ-259B.  Trim for 0 ohms reactance (resonance) 
and ignore swr.

The lines for my 40m 4sq went missing when I returned from TX5C so I used my 
VNA to trim new ones - same method - 0 ohms reactance and ignore the swr.

Both of my 4 squares are optimized for 25 KHz up from the bottom of the band 
but they are still effective across the whole band.  When you use the Comtek 
box the swr at the transmitter doesn't mean much, but you do want to be able 
to observe the power going to the 5th port.

My 30m4sq uses parasitic feed and is a whole different ballgame.

de Paul, W8AEF

ZF2JI/ZF2TA  FO8DX/FO8PLA  8Q7AA  XZ0A  VU7RG  TX5C



----- Original Message ----- 
From: "Stein Roar Brobakken" <s-roabr at online.no>
To: <towertalk at contesting.com>
Sent: Tuesday, April 21, 2009 9:17 AM
Subject: [TowerTalk] RG11 75ohm and 4 square 80m


Hi Guys

I been searching around the internet now for a while, I am about to build a
80m 4 square and and to cut coax for it. I am going to use black RG11 coax.

I have a MFJ 259B to measure velocity factor with.. It's about 10 years
since I did previous time, and now I thinking about to build a 80m 4 square.

So I have a few question for you.
1. Is it any difference measure 75 or 50 ohm ? I meant it should be X=1
if you used it on 75 ohm instead of X=0 on 50 ? this is on a ¼ wl 75 ohm
2. I have a RG-11 75ohm coax I plan to use, but I am not sure of the VF
all my friends says is should be VF 0.66 ?

To use the MFj 259 B ...
What I believe I did previous time I was making cables for a 40m 4 square I
did.
1. to cut the coax on 14,5 meter
2. Solder one connector in the end
3. connected it to the MFJ 259.
4. Measure on 3675
5. Cut until X=1

I meant I saw a footnote into the old manual about "this" way of measure 75
ohm instead of 50 ohm. but I cannot find any reference in any topic on the
net.. I tried to email W8JI, but reply.

The Comtek ACB-4 80m says it need  a ¼ wl 75 ohm coax as I remember. But I
saw here in MFJ 259B manual you can measure 1/4, 3/4,1-1/4, etc.
So does the 4 Square accept if I use ½ wl of 75 ohm to each vertical ? what
will happen?

If I cut one ¼ wl on 3,675 khz is 20,40 meter x 0,66 VF = 13,47 m + 5 %
extra for tuning ? = 14,14 m. it will be too short for get across between
two verticals.

I am worried about the length of this coax since I plan to have the feeding
point of each vertical @ 2.5-3 m above the ground.
I found out that my if I have a 20.5 m x 20.5 m square and the it will be
29,05m across. so then the coax of 13,47-14,14m is not long enough.
29,05 / 2 = 14,525 m.. so the coax will be to short anyway..
What is normal to do in a ¼ wl 4 square ?  the  coax can't be run as  a
string in the air, I wanna have it smooth into the ground....

if I double the length ? it will be about 26,8 +/. Meter.
Will the coax work ok? if I plan to have the feeding point on 2,5-3m above
the ground on 2 x 2"x6" wooden poles on each vertical.


Can anyone tell me How wide is a 80m 4 square in frequency ?


73s Rag LA6FJA
LN5O in contest also:K3RAG SO5RAG