Thanks Alan and others for the useful comments.
My main problem is obstacles when lying out earth radials.. animals,
vehicles and tarmac are a problem in the yard here!
Ideally i should move the antenna down the field and then i would have
chance to run some earth wires shallow in the ground to long distances
but I'll see what i can do with the current setup first.
More capacitance at the top maybe a problem wit the current fibreglass
poles as they are just tapered fishing poles of 10m length, they will
not support the weight of multiple wires unless i go to <0.5mm.. might
be a bit risky in the wind!
I'll try some earth radials today rather than the single rod then think
about top capacitance again.
Rob
On 28/01/2012 00:05, Alan Melia wrote:
Hi Rob it sounds like you must have a loss resistance in excess of 100ohms,
it you can only get an amp with 90W. Unlike Top Band, putting ground rods
under the vertical doesnt help much at LF. Most (55%)of the return current
is collected from under the remote ends of the to wire.
Think about it....this is where the maximum voltage is, and the current
passes through the top capaxity to ground. The bigger the area of the
top-wire/ground acapaitor the less loss in the ground "plate". If you have
poor conductivity ground try running a wire back from the remote groundstake
to the TX ground. If it doesnt make any difference you can remove it
easily. The next step is to incease the top load capacitance. More wire in
the air. The paralell wires should ideally be around 70cm apart minimum.
They can be connected anyhow series, parallel, or meander. Dont worry about
current cancelling, these wires dont radiate anyway. Dont run the top wire
into or close to or over bushes or foliage. I you terminate the top wire on
a tree make the end insulator as far from the tree and it is above the
ground. Even if this shortens the antenna it will reduce the "environmental
loss" Dont run the top wire over roofs and keep the vertcal as far as
possible away from walls. If necessary slope it out to the top wire. The top
wire only needs to be thick enough to support its own weight. Resistance up
there doesnt matter, this part of the aerial is above the effective Rrad (in
the vertical section) so doesnt add to "loss" (even though that sounds
counter-intuitive).
The best way to improve the aerial is to build a simple bridge, not a noise
bridge that wont work at LF, the aerial has to be a measured at the
frequency of use, because the loss is frequency dependent. You require to
use several volts of bridge source drive (to overcome the received signal
pick up) and a tuned detector like a receiver. A good bridge null will be of
the order of 50dB deep or your bridge is not working properly. It will tell
you whether simple quick alterations are improving things before you spend a
lot of time and money "engineering them" properly. You measure the untuned,
(unloaded) wire so it saves risking you PA and keeps the qrm down too. It
looks like a capacitance and resistance in series and the resistance is "ALL
LOSS" the radiation resistance is minimal. So try mods to increase the
capacitance and reduce the resistance emasured.
This is distilled from 15 years of experiments on the band by many of the
pioneers (G3XDV G3LDO, G3AQC, EI0CF, MOBMU, Bill Ashwell US Lowfer, and many
other too) using normal domestic enviroments . Dont be diverted by what the
"professionals do" they dont operate from a domestic premises and use
verticals or umbrellas.......they use earth mats because you need those
under a vertical but they are not really any advantage under an "L" or "T"
unless they are at least 1/8th wavelength in dimensions (300 m square!!).
Bill built an aerial over virgin rock, but he needed a lot of elevated
isolated radials (counterpoise) to get it to work properly. On the very poor
ground, the radials couple to the lossy ground, increasing the loss over the
use of elevated wires.
Some may disagree but what works will depend on your particular location and
ground conditions, and what works for someone else may not work for you. I
sit on top of heathland, very sandy with the water table about 60 feet
down....not exactly ideal. But I got an aerial loss down to around 40 ohms,
in approximatly the same size as you have there (60m top "L")
Good Luck with it
Alan G3NYK
----- Original Message -----
From: "Rob, M0DTS"<[email protected]>
To:<[email protected]>
Sent: Friday, January 27, 2012 7:52 PM
Subject: Re: LF: Conds
My TX is now working all be it fairly QRP 90W ;-)
Antenna is 53m horizontal at ~9m high.. a droopy 'T' single strand
copper all the way.
I'm also receiving on this antenna too with much better results than the
loop i have.
Problem is going to be getting enough routes to earth, my last attempts
were getting ~1A antenna current with the same power but it looks much
worse on G4WGT's grabber than last time so i guess it's less than 1A.
Will do some more improvements over the weekend.
Rob
M0DTS
On 27/01/2012 18:56, Stefan Schäfer wrote:
Hi Rob,
Thanks for the report.
Oh yes, looking forward to see you on the band (or hear?).
What will you use as the TX antenna?
73, Stefan/DK7FC
PS: Conds (local) are excellent again this evening! Worth to come to
QSO mode ;-)
Am 26.01.2012 19:29, schrieb Rob, M0DTS:
On 25/01/2012 23:28, Stefan Schäfer wrote:
LF,
Excellent conds on 137 in a range of 2500km! Signals show up in RN,
4X, TF, YO at good strength. Worth to try!
73, Stefan/DK7FC
Stefan.. vy good tonight again too, you're 579 here.. best I've heard
you!
Still trying to get time to re-assemble the tx and make some
transmissions.......
Rob
M0DTS
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