[TowerTalk] V-Beams and Antenna Tuners

[Jim Reid]

Aloha,

Back in June I wrote:

"I've had Paul's XMatch now for many months;  am super pleased
with it!!  I also have,  for my V-beams, planned for set-up
as soon as the darn trade winds calm down -- up to 36 mph this
afternoon -- an "old" E F Johnson KW Matchbox.  Now that IS
an ugly, very big box!!!  Guess it looked beautiful next to
the desk sized Johnson KW AM rig around 45 years ago!

But if you want the best HF band tuner,  1.8 thru 29.7 mHz,  buy
Paul's tuner.  It might take awhile to get one,  he makes 
them one by one,  carefully obviously!"

And last week I posted a short note about my physical
exhaustion from getting the V wires out and up.  Well,  at
last, success.  All four 600 foot long wires are out;
the V's are wired,  the switching box installed up at the
feed point,  and THEY WORK!!!  Will not here describe how
I missed the CQWW 'test,  but did get the V's up and operating
this week end,  also attended the Hawaii Chapter annual QCWA
business meeting down in Hilo.  Probably should have stayed
at home,  however,  as I was elected President for the next
two years 98, 99!!  Just what I needed: railroaded.

Anyway,  again I have questions.  I would like to use Paul's
XMatch with the V's, primarily to cover 160,  which the
Matchbox cannot,  but also I find the Matchbox weird to 
operate!!  I have only gotten it to match to the V's on
80, 40 and 30; cannot get below 3:1 or so on the higher
bands. The Johnson box uses a transformer coupled few
turn link to couple the rig input/output to the tuner's
secondary coil.  The coil is tapped using more turns on
the lower bands,  fewer on the upper. A large dual
section cap tunes the secondary to form a parallel resonant
tank at the frequency of operation.  Then a very large
split differential capacitor is  in parallel with the
resonant tank;  in theory,  I think,  this forms a
capacitive voltage divider/ impedance matcher to whatever
impedance the balanced transmission line is presenting
to the tuner.  

In practice,  however,  what seems to occur is that,  while
it may do what is described above,  it also de tunes the tank!?
The result is a strange tune/detune/readjust the main tank,
try a new voltage divider position, detunes,  etc.,...,etc!

This does not make for rapid QSY!  And with the high impedance
presented by the V's, a given tuning is only good for a few
kHz above and below that tuned. A few V beam impedances,  as
calculated by AO:

1.830 mHz	181 - j965 ohms

3.515		157 - j611

7.005		208 + j234

10.105		173 - j272

14.005		327 - j823

21.295		236 - j540

28.495		168 - j116

so, vswr's with 450 ohm twin lines are not excessive.

I am using the Wireman's 450 ohm twin line from the Matchbox.

Now,  can I buy a balun to use with the XMatch and the V's?

The Amidon 9:1 for V's and rhombics or a 4:1 or what?

And should I be able to tune the bands above 30 meters
with the Matchbox?  Maybe the tune is so sharp I have not
yet found the "sweet spot".

First on-the-air tests yesterday  on 40 meters down to the 
Big Island,  about 340 miles a little East of South-East,
shows a gain of about 6 dB over my inverted V 40 meter
dipole, by the V that is aimed roughly down the island chain.
That is worth the effort if it holds up for DX. Also should
mention,  that when tuned,  I can switch among the four V's
and the match remains essentially flat maybe going from 1:1
to 1:1.2.

73 and thanks for any thoughts on this tuning issue.

Jim,  KH7M
On the Garden Island of Kauai
where it has been raining while the Dow goes down 550+ points.




--
FAQ on WWW:               http://www.contesting.com/towertalkfaq.html
Submissions:              towertalk@contesting.com
Administrative requests:  towertalk-REQUEST@contesting.com
Problems:                 owner-towertalk@contesting.com
Search:                   http://www.contesting.com/km9p/search