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Re: LF: Puckeridge Decca station - Big & small antennas

To: [email protected]
Subject: Re: LF: Puckeridge Decca station - Big & small antennas
From: "g3kev" <[email protected]>
Date: Wed, 17 May 2000 18:33:15 +0000
Organization: Netscape Online member
References: <[email protected]>
Reply-to: [email protected]
Sender: <[email protected]>

James Moritz wrote:

Dear LF Group,

This weekend, I am hoping to make another expedition to the
Puckeridge Decca station, which has been made available again
through the good offices of G3JKV and the Crawley club.

The main motive for this expedition is to do some back-to-back
comparisons between a big antenna (the 100m Decca mast) and a
small, amateur-type antenna (an inverted L about 9m high and 50m
The large mast is vertical and the smaller inv L is more horizontal than
vertical. So your comparison between the two antennas will not be valid.
To my knowledge no one in the past has disputed that a large and small
vertical would have the same radiation PATTERN. What has been said is
that the intensity of the signal radiated from a large vertical is
greater than a short one. On receive the signal voltage collected by a
large vertical is greater than the small vertical.
One must be precise and accurate when making comparisons. Some
observations in the past round up or down to the nearest 6 db. One must
realise that represents a significant power difference on transmit not to
mention receive.
There will be other factors to consider between the antennas like tx
signal take off angles, also the response on receive for each antenna to
high or low angles of signal arrival.
Then comes the ground radial systems for each, they must be separate and
identical and each antenna must be located far enough away from the other
to avoid coupling.
If all the above factors are not met then it would be a useless exercise.


The idea is to set the antenna currents so that the same
effective radiated power should be obtained from both antennas,
and then see how signal strengths compare.

There has been some vigorous debate lately about the advantages
and disadvantages of big and small antennas, with some holding
the view that big antennas have superior radiation patterns to small
ones, and some holding the opposite is true. The aim of the
experiment is to put this to a practical test. We did this before
during the previous trip to Puckeridge, but due to time pressures
and the dreadful weather, relatively little operating was done using
the two antenna setup. The results then were that the two
antennas gave roughly the same results, when transmitter power
was set for same ERP from each antenna (this meant feeding
about 500W into the small antenna, and less than 0.5W into the
main mast!).

For this weekend, I have put together a tuner/attenuator circuit that
will hopefully allow changing between the antennas with a single
switch, making the comparison much quicker and more direct. I am
hoping this will enable lots more people to take part. The level of
ERP will be very roughly 100mW for both antennas.

The plan for the weekend is roughly this:
Friday evening - Install gear
Saturday morning - get working, and obtain comparative reports.
Later on Saturday - run station at 1W ERP from main antenna
Sunday - open to suggestions; I have notified RA in case anyone
would like to try 73kHz QSO's.
Sunday afternoon - pack up and go home.

Due to this being a small-scale expedition, I will probably stick to
normal CW, but might be prevailed upon to drag my PC across a
field if people are really keen to hear some QRSS.......

Of course, nothing ever goes entirely according to plan, but it
should be fun. Any QSO's, listener reports or measurements would
be welcome. If you have any suggestions, please let me know.

Cheers, Jim Moritz
73 de M0BMU

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