Hi Dick, Cynical responses to your kind of query in the past have been in
vein of "Buy another house with more space"
Seriously you are up against some basic laws. Empirical test suggest that
the minimum spacing to get some reasonable increase from the second top wire
is 70cm Three wires with a max spread of just over a meter might give some
more. You can use a ball-park figure ( as out TA friends would say) of 6pF
per metre so 2 runs of 7.5 metre equates to about 90pF the other 45pF you
are seeing is probably the vertical to ground. To get another 50pF you will
need to get out another 9 or 10 metres. It can go anywhere connected at any
point but it should be over open ground and not over trees or roofs. It can
even be in a meander observing the 70cm spacing. The wire only needs to be
thick enough to support its span. It doesnt make a lot of difference where
you feed it....centre or end, the top capacity is the same. In fact you may
get a slightly higher Rrad by end feeding it as an inverted "L " To get the
maximum effective height you need a run of top wire from the feed point of
about 50% longer than the real height of the top. I dont remember seeing a
calculation for a meander top wire.
The increase in capacity will reduce your tuning inductor, but unfortunately
with so low a total capacity you will probably have a ground loss in excess
of 200ohms regardles of what you do with ground rods. This means the ERP
from your site will be quite small. In this situation you may radiate more
with a loop despite its inefficiency at that limited size, it does not
suffer from loss from surrounding structures or trees. Dave Sargent was
unable to run a inverted L and used a loop with low power (~ 50 watts)
Cheers de Alan G3NYK
----- Original Message -----
From: Dick <[email protected]>
To: <[email protected]>
Sent: 11 August 2006 20:45
Subject: LF: antenna: how to get max. capacity
Hello all,
In order to test my 136kHz TX at home, I just have installed a toploaded
wire in the backgarden.
It measured only 135pF, which is too low, because need around 10mH to get
that running on 136......
System is as follows:
2 wires each 7.5m long mounted parallel (spacing 30cm), at both ends
connected together, and in
the center connected together.
>From the center an about 7m long wire goes vertical downward to the TX.
Any suggestions how to get more capacitance in the air?
I am limited in options, because the topload has an maximum length of 7.5m,
and maximum width of 1m.
Some ideas I have:
-opening up the ends
-adding a third wire in de middle, feed it there and then bending the rest,
a kind of Z-shapped topload, center fed.
Would be happy to get 40 - 50pF extra up in the air.......
73
Dick, PA4VHF
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