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RE: [TowerTalk] Ground radials/elevated radials

To: "'Tom Rauch'" <>,"'Yo3ctk'" <>, <>
Subject: RE: [TowerTalk] Ground radials/elevated radials
From: "Paul Playford" <>
Date: Sun, 24 Oct 2004 01:53:24 -0000
List-post: <>
> The same would hold true for elevated radials?

> I am designing a 4-square for 80m. Full size verticals. Comteq
phasing/switching unit. But I need elevated radials
> for practical reasons (the land below is used for small-size agriculture).
I am not sure how  many and how long
> radials are needed for each antenna. I am not after the ultimate system,
only one with decent results.

> Thank you for any suggestion. Best regards,

> 73 de Mike, YO3CTK (YR7M in contests)

Perhaps I missed some previous messages in this thread?

In reading the above I concluded that on the ground radials are not a
consideration.  And he states that he is not after the ultimate system, only
one with decent results.  So his question is how many and how long?

The point I am trying to make is that a 4 squared phase array with 4
elevated elevated radials will work, the length of elevated radials is
critical and that coax manufacturers are not always truthful when they state
velocity factor.  +/-10% is the tolerance I have found and in my opinion
that is not close enough when you are cutting impedance matching
transformers and phasing lines.

Unfortunately I did not explain the method I used to determine the length of
my radials.  I erected two radials and fed them in the center as though it
was a dipole and measured the resonant frequency (0 ohms reactance).  Then I
adjusted the lengths of the radials to resonate at my desired frequency.
Then I installed all of the rest of the radials, erected all of the vertical
elements (all with identical construction) and attached all of the feed
lines.  I fed power into the system and observed the frequency at which the
minimum amount of power went into the 5th port's dummy load for each
direction.  Ideally the observed frequency will be the same for each
direction but we don't live in an ideal world so I did a little fudging,
came up with a compromise length and adjusted all of the vertical elements
to this length.  The radials come out slightly short due to their proximity
to earth which means the vertical elements will be slightly long.

Beyond that he has gotta look at his own conditions.  Can he install more
than 4 (or 8) elevated radials?  Are his neighboring hams (within 300 Km)
running a 3 element yagi at 48 meters or just a 2 element yagi or dipole at
24 meters?  I am of the opinion that if the hams in his area are all running
3 element yagis at 48 meters he will not be able to break their pileup.  But
if they are running the low 2 element yagi or dipole they will have a tough
time breaking his pileup.  This with 4 elevated radials under each driven
element.  Regardless, he will still observe between 20 and 30 db of front to
back and front to side ratio, which is great if he has a noise coming from
one or two directions.

As a point of curiosity I did measure the feed point impedance at the base
of one vertical (all other lines connected) and it is very close to 36 ohms
when the reactance is 0 (ignore the frequency here).  I have checked the
calibration of my MFJ-259 and it is correct at 0 ohms and 50 ohms so I
believe the 36 ohms is very close.

And then there is anecdotal evidence.  I helped a friend install his 4
element full size 40m yagi on his 100 foot tower.  That evening I spotted an
African country and almost instantly I got a 'T' message from him "Can you
hear him?"  Isn't that what we want?

de Paul, W8AEF

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