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LF: Report on Decca ant. tests 11 April 2000 - G3WSC/P

To: [email protected]
Subject: LF: Report on Decca ant. tests 11 April 2000 - G3WSC/P
From: "DEREK ATTER" <[email protected]>
Date: Wed, 12 Apr 2000 14:33:41 -0400
Reply-to: [email protected]
Sender: <[email protected]>
Re the initial on-air tests using the de-commissioned Decca mast at
Puckeridge yesterday :

  We were rather later than planned getting on air on 136Khz due to delays
and heavy traffic on the motorway round London and also due to having to
hump the gear in heavy rain from the car 200m to the antenna hut! The main objectives of this initial session were firstly to
establish that we could actually tune up the antenna on both 136 and 73 khz
with our matching and loading coils given that the base impedance looks
like 3750pf in series with about 5 ohms resistive. The height of the
antenna mast which is similar to the Eiffel tower in shape,  is 100m
(325ft) with an extensive earth mat system. The second objective was to radiate as near as possible 1W ERP on
both bands given the known efficiency of the antenna sytem at several
frequencies (including the original very large loading coils) correcting for the somewhat higher loss of our quite small loading coil/variometer And then finally make some QSOs as time permitted given that we had a 2
hour drive home!.

We had very little trouble in loading up on 136khz apart from a PTT line
problem at the start  between the transceiver and TX.  The transmitter was
a linear amplifier operating in class B with a low pass filter at the
output and the sinewave output shape monitored by an oscilloscope.  At the
base of the loading coil to ground was a tapped toroidal auto transformer
with an optimum  impedance transformation ratio of 10 to 1 (ie 50 ohms at
the TX input and 5 ohms in the base of the antenna system.) we also needed
a 5000pf HV capacitor (25Kv wkg !) in series with the antenna to enable our
variometer to tune.
  The estimated efficiency based on the original measured Decca figures
was 10% and we therefore ran 10 watts at 136khz into the matching
transformer in base of the antenna monitored by a forward and reverse power

At our first test call we immediately had a pile up and had a number of
QSOs before it was time to change over to 73Khz to try to tune up on that
band. Our apologies to those we were not able to work in our first spell
but our main operating sessions are planned to be on Fri/Sat/ and possibly
Sunday 14/15/16 April  we will post info on the planned operating times
soon as possible - depending on whether or not we get our higher power
licence variation in time.

Tuning up on 73khz proved unfortunately to be much more difficult than we
expected with the coils etc which we had taken with us and it took us a
long time to get going on that band due to interaction between tuning and
matching adjustments although we did eventually get an acceptable match and
tuning position. We were joined during the evening by Jim M0BMU and Mike
G3XDV who live fairly locally and  who helped to solve the matching
problem.  We did then manage to work Dave G3YXM on 73Khz but unfortunately
by then it was time to pack up and go home arriving home at 23-30 UTC (00-30 local ! )
Stations worked :

136Khz :  DJ6FU   (559 sent ,  569 recvd ) ;  ON6ND (589 sent, 599+ recvd);
DK9DX  (579 sent,  599 recvd)

                  DL3FDO  (569 sent,  559 recvd) ;  ON7YD  (579 sent,  599
recvd)   G4GVC  (599 sent  599+ 50dB recvd !!)

                  DJ9IE   (579 sent  599+10dB)    PA0SE  (579  sent,  599

73Khz   :   G3YXM  (579 sent,  589 recvd)

It was noticeable that the Loran QRM although much stronger in absolute
terms, was lower in relation to incoming amateur signals than at the home
station in Crawley further South in IO91VC.  Later there was also a high
level of QRN due to a rain storm from about 1800 UTC especially on 73 Khz. Thanks to those people who have sent signal reports via E-mail etc. and
also gave these two old codgers ( sum of our ages 148 !!) verbal support.

               73,  de   Lech,  G3KAU & Derek G3GRO

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