[TowerTalk] HF Vertical on tower? RF Spill Over
K7GCO@aol.com
K7GCO@aol.com
Sat, 22 Jul 2000 14:22:31 EDT
In a message dated 7/21/00 5:26:22 PM Pacific Daylight Time,
w8ji@contesting.com writes:
<<
Verticals on higher bands always work better up in the clear than
they do on the ground in a cluttered environment. That isn't
necessarily true on 160 and 80, where surrounding structures are
short in terms of wavelength and ground losses are less.
I've had very good success with verticals above yogis, and intend to
install a nice trap vertical on top of my rotating tower as soon as I
get the other antennas mounted.
> The main difference obtained by differences in height will depend on
> how much RF spill over on to the coax shield, mast & tower. It
increases the
> angle of radiation. I've mounted 1/4 WL verticals on top of yogis using
> the DE as it's ground plane, 1/2 WL verticals with no RF spill over on
> towers and also over yogis. They work great high off the ground when
they
> don't have any RF spill over. I'd add the donuts (lots of them) to the
coax at the
> bottom of the and give it a try. Buy the Palomar RF current meter and
> measure it before and after.
Unless you insulate the base of the vertical (where the shield
connects) from the tower, adding sleeves on the feedline won't do
much. All that happens is the current shifts to the tower.
Some ground planes use insulated mounting brackets with a choke
balun, and the radials all float for RF. This prevents the feedline and
supporting structure from having common mode currents.
Another scheme surrounds the mounting structure and feedline
with an open skirt. The end of the skirt has to have a reasonable
amount larger than the structure inside or it generally won't choke
well.
> The higher they are the more the tower predominates and the less affect
> the antenna has. One set of radials doesn't stop it all. That's why
the
> AEA Isopole has the least RF Spill Over. The 2nd 1/4 WL Sleeve kills
it.
> The 2M Ringo was bad, they tried to clean it up and put the extra
> decoupling stubs in the wrong place.
Stubs (radials) are always less effective than a 1/4 WL decoupling
sleeve. I'm not so sure Cushcraft put the radials in the wrong spot, I
am sure four radials are not enough to decouple a mast.
73, Tom W8JI
w8ji@contesting.com
In regard to the RF shifting to the mast with sleeves or whatever on the
coax shield, that is true. I failed to mention that I used a fiberglass mast
which rejects the "Dreaded RF Spill Over" quite well. I've tried to keep
Posts short but I end up having to explain further at times to correct
misconceptions, opinions or revisions of what I said.
In regard to radials being less effective than sleeves, it depends which
way the sleeve points. The open bazooka sleeve that points up and at the
feedpoint is very effective. However if the 1/4 wave sleeve open end points
down as used in the 70 ohm Sleeve Vertical used way back in the 30's and
frequently shown in the literature, it really swamps the mast with RF. They
were always fed with 70 ohm coax, the higher they were mounted the worse they
worked (Ringo also) and may have started a concept of high verticals don't
work well. Running 4 radials close to the mast does the same thing due to
proxsimetry, capacity coupling and about 70 ohms results at the feedpoint.
Horizontal radials give about 35 ohms. Radials at about 30 degrees from the
mast creates about 50 ohms and a fair amount of RF Spill Over. There was a CB
antenna made that way. I've duplicated all this with 2M models for talks at
clubs and used neon lights as RF sniffers to show the levels of RF Spill
Over. I also used a small loop, a light bulb and a trimmer capacitor for a
very sensitive sniffer. Neon lamps have a threshold voltage before they
ignite.
Now here is a concept that will open ones eyes not seen by very many. The
RF that gets by the radials is determined by the length of the mast/coax
shield. If it is a 1/4 WL multiple and essentially at ground potential at
the bottom, a Hi-Z is reflected at the top just below the feedpoint and is
almost an automatic choke. If the mast/coax length is a 1/2 WL multiple and
at ground potential at the bottom, a Lo-Z is reflected at the top and it
COMPETES with the radials for RF based on Ohms Law. RF doesn't know it's not
supposed to do that but it does anyway. The RF Spill Over is high enough
that 1/4 WL below the feedpoint the voltage level and Z is very high and just
placing your hand there will affect the SWR if the RF Spill Over is high
enough. Your hand on the coax/mast just below the feedpoint will have no
affect on the SWR in the Lo-Z Hi-Current area. Run the test yourself if you
have any doubts.
Now it occurred to me that the way to make 1/4 WL radials really effective
and the Lo-Z they create at the connection point, is to attach them at a Hi-Z
point for decoupling purposes. They will absorb a lot of RF there. So 1/4 WL
below the feedpoint where the RF Spill Over Z was at its highest voltage and
impedance, I connected a radial. PRESTO! It cleaned the mast of virtually
100 % of the RF Spill Over. The tip of this 1/4 WL radial was as hot as the
radials above. You could literally hear the RF sucking sound. Would you
believe the mast with the coax inside was stone cold with icicles forming on
it in the summer. The obvious conclusion is that the 2nd set of radials or
just even with one of them 1/4 WL BELOW the Lo-Z feedpoint will effectively
cool the mast.
Now this is the principle behind the AEA Isopole. It uses 2 1/4 WL cone
sleeves below the feedpoint. It could have been 2 sets of 3 or 4 radials. I
showed a 2M version of what I just described to Dr Don Reynolds W7DBA of the
U of Wash in 1958. He used 2 sleeves on the design of AEA 5/8 WL Isopole
about 15 years ago as they look better he thought. Looks sells. It's the
source of the name "Isopole" as the sleeves electrically isolate the vertical
from the pole. Now you know the rest of the story. It's the first
commercial vertical design that really addressed the problem properly.
The QST 52 2M vertical article by Remington Rand of Shaver Fame (look it
up) I referred to earlier was originally a 70 ohm Sleeve vertical guilty of
"Excessive RF Spill Over." It was a very poor performer with a high angle or
radiation. So he was very clever and put the RF Spill over to work. Its
done it's damage long enough. He added 3 more 1/4 WL sleeves 1/4 WL below
each other on the mast. Each one creates a 180 degree phase shift inside the
sleeve and the RF coming out the bottom is the same phase as the RF above it.
The end affect was he ended up with 4 1/2 Waves in phase excited at the top
which gives broadside gain if that's all there is. Each one had a bit less
RF radiating from it as a result of RF losses and resulting radiation from
each section. 1/4 WL below the 4th sleeve he added 4 horizontal radials to
stop most of what ever RF was left. This was a brilliant vertical
design--finally!
In a picture caption the claim was made of 4-5 S units improvement. How
can this be? The actual gain improvement from the "tips of the major lobes"
was about 6 dB. The single sleeve vertical lobe was at about 45 degrees as I
remember. But the tip of the major lobe was mostly on the horizon where
everyone assumes it is. There was indeed significant difference in the gain
directed on the horizon from the high angle lobe and the 6 dB gain "educated
lobe" mostly on the horizon where on 2M it does the most good. I still have
the very antenna used in the 52 QST article. It was tested on the U of Wash
antenna test range and helped inspire the Isopole. I plan to install it
along with the single sleeve vertical just to give demonstrations on the air
of what I have just described for all the Nay-Sayers.
One reason this concept has evaded so many mfgs for so long is that in
their test set ups I've seen, they used a very short mast horizontally
oriented with the test vertical mounted on it which was rotated for the H
plane or the vertical pattern to check the gain. If they had used say a
40-50' mast horizontally oriented with the test antenna mounted on it and
they rotated it to check the H plane, they would have seen the affect of the
RF Spill Over on the equivalent of higher towers typically used for the
design they had. I did the equivalent of this with a 450 MHz antenna and HT
receiver using a signal from a horizontally polarized source. The pattern
wasn't broadside as one would expect.
Since all the verticals worked about he same due to the same reason, no
one suspected the vertical pattern for most of them wasn't on the horizon
unless they had a way to check it. Their performance seemed normal. Even if
they found this out not too many knew what to do about it. RF can be very
evasive. I'd like to develop a RF Fog in a spray can that would illuminate
in RF fields.
Fortunately most verticals used on repeaters high on a mountain had short
masts as any pattern lift off was detrimental to performance. However one
repeater installation pulled a big dumb dumb. They mounted the vertical
adjacent to a metal tower with horizontal fiberglass horizontal supports for
strength as there were high winds and ice loading. The vertical mounted in
the normal way wouldn't take the loads. The vertical had a good broadside
pattern--right into the supporting tower. I finally got them to attach it to
a telephone pole and it worked much better. Some commercial mfgs actually
have designs that the free space pattern tilts down for repeater use. I got
another to add another set of radials 1/4 WL below the regular radials of the
antenna they were using and it improved the performance much to their
surprise--not mine.
Now in regard to the Ringo 2M vertical. The original design had a large
amount of RF spill over as there was absolutely nothing to stop it. It was a
Hi-Z end feed vertical with a tank circuit (the 1 turn ring) and a 50 ohm tap
for the coax. After AEA came out with the Isopole, CC finally saw the light
and added what they thought was equivalent. They added 3 radials below the
Ring. Unfortunately they didn't understand what they were doing on the one I
saw and added them 1/8 WL below the Ring--not 1/4 WL for maximum decoupling
affect. I have a Ringo here where I demonstrate this. This is the basis of
my previous statement that "they put them at the wrong place." The statement
made above "I don't think CC put the radials in the wrong place" is without
any foundation or tests to verify it. Further more 4 or even one radial will
cool a mast if it as at the right place below the primary radials which are
mostly for resonance and preventing RF Spill Over secondarily. I say
secondarily as they either work well or they don't depending on the Z of the
mast determined by it's length that competes with the radials. RF follows
the lowest Z path based on Ohms Law--not the designer desires or assumptions.
It doesn't know how long the mast is either--it sees just the Z of the mast
where it originates with the radials. RF is just like water. It will flow
into the lowest resistance and/or a lower plane due to gravity.
I hope this provides sufficient technical background for my statements.
Everything I said is verifiable with the various 2M verticals. I plan to
make a video tape of this, pass it around and it eliminates the need to carry
these antennas around for the demonstrations. I hope this answers all the
questions and provides some basic insight into this very simple concept that
has been very evasive. I won a bet with a friend that I knew I'd have to
explain this in detail which I was glad to do. That is what TT is for. As I
said before only 2 major articles have been printed on this very evasive
subject. Perhaps this could be considered the 3rd one. I only wish the
various patterns I have could be shown. When I get set up I'll have ways to
show this, a lot of other data and pictures for those who request it. I'm
getting a digital camera and film scanners for up the 4x5 negatives K7GCO
--
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