[Top] [All Lists]


To: <>
Subject: [TowerTalk] RADIAL QUESTIONS
From: (CQ K8DO)
Date: Thu, 18 Dec 1997 14:03:52 EST
I have put significant amounts of blood, sweat, and tears into vertical arrays
for 160, over the years... As a result I have definite opinions in this area
of antenna practice...

>Since the sides of the square are 35 feet, the
>radials extending inside the square would intersect. The theory is that this
>can cause undesirable cancellation of currents in the radials, so the
>standard procedure is to bind all the radials inside the square to a pair of
>bus wires that cross in the middle of the square 

First, I have a problem with the concept that radials crossing each other
cause undesireable cancellation of the currents... And with the concept that
connecting everything to a central buss bar cancels this cancellation....  
The first thing a common buss does is to concentrate the currents into a
smaller area raising the I^2R ground losses, especially for an on the ground
system ... The whole idea of an extensive radial field is to spread the
currents over a larger area thereby decreasing the current per square meter
(or what ever units you prefer) ...
And, I have a definite bias against rolling out a length of bare wire on the
ground and thinking that it has some specific electrical length.... the
electrical length of that wire is unknown and unmeasureable... Dirt is a
slurry of ionized salts, water, metals,and carbon, impregnated through a dough
of nonconductive material - forming a partially conductive plane... With the
addition of bare radials all you have is dirt with a high concentration of
metal in long strips.... The RF current, initially finding metal strips to
have a lower surge impedence than the adjacent dirt near the antenna, will
preferentially flow along the wire, but current will be leaking laterally
(shorting) into the ground between the radials as it goes... At the same time
the dirt is shorting the bare wire back towards zero volts of RF, thus
destroying any resonance the installer might have been counting on... 
This whole scenario is why I do not use bare wire for radials... I do not have
any chance at all of establishing resonance in this half of the antenna
system... All you can do is add more radials hoping to increase the percentage
of metal in your dirt...  Certainly, 4X8 sheets of pure copper covering the
ground for a few hundred feet will do even better, but is impractical...

and have been
operating happily on 40M ever since (the antenna is very broad; the SWR
hardly budges across the entire band.)
This should be a wake up call.... A single tapering, quarter wave , vertical
element is not going to be that broad...

<multiple snips ending with>
When I tried the input impedance measurement, I found that
the point of lowest impedance was 25 ohms at about 7.300! When I checked the
impedance at 7.150, it was 37 ohms.
A 50 ohm coax is only flat when terminated in a 50 ohm load... A random length
of coax when terminated in non 50 ohm load is going to act as a voltage and
phase transformer, and give you problems at the feed end... Your
antenna/ground-system is resonant at 7.300 (the point of lowest impedence is
the resonant frequency, when measured at the antenna terminals)... Now, you
need to trim the antenna until it is resonant where you want it... Then you
need to measure the impedence, and with this information you can design the
matching device... I prefer a wound transformer / tapped coil / etc., over
quarter wave lengths of coax, but they all work when properly designed...
The way I prefer to tune a vertical is to first hang two 1/4 wave elevated
radials - 3 or 4 feet high is fine for 40 meters -  fold them to fit the
space, if needed..(shorter radials are ok, just load them until they are
resonant)... Then using the RF-1 drive the two radials at the center as a
dipole antenna, and adjust their length until they are resonant at your chosen
frequency (yes, the impedence at resonance will be a few ohms... Not to
worry)... Now reattach them as radials and drive the vertical element against
the (now) tuned radials... Adjust the vertical length until is is resonant
where you want it... voila, you are done except for the matching network - and
I guarantee it won't be flat across the entire 40 meter band... 
You HAVE to detune (or take down) all 40 meter vertical antennas within 2 or 3
wavelengths of the one you are working on, or they will couple and you will go
mad trying to get the system to behave 
(actually it can't be done if the element you are adjusting is resonance
coupled to another antenna because you have coupled tuned circuits and they
will exhibit pulling of the frequency depending on which direction you are
tuning as you approach one {or more} of the resonance points).. 
When I tuned my 4 element array it required many trips back and forth to
isolate the feed points for other 3 elements, especially with three iterations
around the circle...

I do not use ferrite beads over the coax... I prefer a wound choke balun on
the end of the coax to choke the common mode current....

Cheers  ...  Denny

FAQ on WWW:     
Administrative requests:

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