[TowerTalk] Outboard tuner

[K7GCO@aol.com]

In a message dated 6/27/00 5:15:44 AM Pacific Daylight Time, n4kg@juno.com 
writes:

<< 
 I expect your filter is designed for 50 ohm terminations so
 it should be between the transmitter and antenna tuner.
 
 Using a very short antenna with coax will result in VERY
 HIGH SWR which will cause the coax losses to increase.
 
 Lower loss feedline such as LADDERLINE will minimize
 feedline losses.
 
 Dropping the feedline straight down and feeding the 40M
 dipole as a top loaded vertical on 80 and 160M would be
 even more efficient.  Of course you will need radials,
 preferably 30 or more.
 
 de  Tom  N4KG >>

    When tuning a dipole as a top loaded vertical and the end of the feedline 
is 1/4 or 1/2 WL long (dipole 1 WL long), the Z is high around 2500 ohms or 
more.  The tuner must be able to match this and typical tuners in rigs may 
not be able to match this.  The need for a low loss ground system or radials 
to complete the resonance is not necessary.  The antenna is already a 1/2 or 
1 wave long and is to be voltage or Hi-Z fed.  If it is a 1/4 wave multiple 
and Lo-Z, you need a good ground preferably of radials instead of just a 
ground rod.  If a lossy ground of RF resistance becomes a value compared to 
the Radiation Resistance, it lowers the efficiency of the feed system.  The 
RF current flow through this resistance results in a significant power loss.

When the feed Z is 2500 ohm or more, the RF ground resistance of a lossy 
ground rod is so small compared to 2500 ohms, the efficiency is still 
maximum.  The current flow is very low in Hi-Z feedpoints.  I and others have 
had success with just ground rods and radials when feeding top loaded dipoles 
but never had a comparative installation side by side to compare.  I will do 
this very test in SD.  I will do it in Eznec.  I'm eager to see what angle 
change there might be with 1/4 wave and longer radials does.

I've used this antenna since 1946.  W7WIP of Bremerton, Wa was the first I 
heard using it.  He could quickly switch between it and the conventional 
dipole configuration.  On 75M to SD there would be several S units 
difference.  It's a great antenna and Hi-Z voltage fed antennas reduce the 
necessity of an extensive radial system.  I will have a 4 Square on 160M and 
use top loaded !/4 waves just so that I can voltage feed the verticals.  I 
like the idea of an upside down current loop as it gets the strongest current 
field far away from the ground and surrounding objects.  I won't have 
surrounding objects in SD, however.  When I used a top loaded vertical on 
75&40M here in Seattle the DX called me.  I had a 40M 8 radial system.  

Conventional wisdom would suggest using radials with voltage fed antennas.  
But for a test I've suggested to many they can operate with a ground rod 
connection for the tuner whatever it is (I and others used L networks not 
remotely tuned but ideal).  It worked so well no one felt it necessary to add 
radials.  The L Network well tune or match without the ground connection but 
it helps to cool the coax shield going to the shack.  One guy had no ground 
rod and the coax was buried which tended to cool the shield with no RF in the 
shack he could detect (the ultimate of simplicity).  Another was on black top 
and another on a roof and still had no RF problem in the shack.  Perhaps the 
coax lengths were ideal in some way.  On a roof I'd sure add at least 4 
radials.  To add radials or not over a ground rod when voltage feeding is 
another concept that needs final confirmation.  After over 50 years of using 
this antenna, this is one concept I will finally have a comparative 
opportunity to resolve on 6 acres in SD.

I had an article on this antenna in CQ about 40 years ago called the "10-90 
Degree Vertical" for 75&40M.  The 10-90 Degree bit refers to the 2 angles of 
radiation possible when voltage and conventional feeding a 75M dipole with a 
1/4 WL feedline--coax or open wire line.  I had all kinds of favorable 
response to it even years later.  W7DND had great success with voltage fed 
verticals with NO RADIALS on a slat water beach on 20-10M.

QST had an article in the late 40's on this antenna where they also had a 
tuner circuit that created the Turnstile polarization. The polarization 
rotated between vertical and horizontal at an RF rate and this is very 
effective on transmit for contests or any time.  It helps reduce QSB on 
transmit and receive also as most of it is from polarization shift.  As I 
recall radials weren't even mentioned but a ground connection was shown.

Radio Mag had a similar article in the 40's also.  They are in my files and 
packed so I don't remember the exact dates.  This is a great antenna for 
160-40M and it can be tried quickly with just a ground rod to cool the coax 
shield.  I recommend as does N4KG to then add radials--and compare.  Be 
surprised for a surprise.

I have a joke I seriously tell about ground connections (on paper) shown in 
the magazine articles and balanced feedpoints fed with coax without a balun 
and perfect patterns are always shown as if there was no RF Spill Over--on 
paper.  The reason is it is shown on non-conductive paper.  If mag articles 
were printed on aluminum foil there would be RF Spill Over on the shield and 
the resulting patterns would be shown.  They would have "RF Reading 
Feedback."  When I can keep a straight face they tend to believe it.   K7GCO

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