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Date: Sat, 28 Jan 2012 10:31:17 +0000
From: "Rob, M0DTS" <rob@m0dts.co.uk>
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Subject: Re: LF: Conds
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Thanks Alan and others for the useful comments.

My main problem is obstacles when lying out earth radials.. animals,=20
vehicles and tarmac are a problem in the yard here!
Ideally i should move the antenna down the field and then i would have=20
chance to run some earth wires shallow in the ground to long distances=20
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=20
poles as they are just tapered fishing poles of 10m length, they will=20
not support the weight of multiple wires unless i go to <0.5mm..  might=20
be a bit risky in the wind!

I'll try some earth radials today rather than the single rod then think=20
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 100o=
hms,
> it you can only get an amp with 90W. Unlike Top Band, putting ground ro=
ds
> under the vertical doesnt help much at LF. Most (55%)of the return curr=
ent
> 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 h=
ave
> poor conductivity ground try running a wire back from the remote ground=
stake
> 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=
=2E
> They can be connected anyhow series, parallel, or meander. Dont worry a=
bout
> current cancelling, these wires dont radiate anyway. Dont run the top w=
ire
> into or close to or over bushes or foliage. I you terminate the top wir=
e 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 "environme=
ntal
> 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. Th=
e top
> wire only needs to be thick enough to support its own weight. Resistanc=
e up
> there doesnt matter, this part of the aerial is above the effective Rra=
d (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 n=
oise
> 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 sign=
al
> 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 sp=
end a
> lot of time and money "engineering them" properly. You measure the untu=
ned,
> (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 t=
he
> 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 ove=
r the
> use of elevated wires.
>
> Some may disagree but what works will depend on your particular locatio=
n 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 o=
hms,
> in approximatly the same size as you have there (60m top "L")
>
> Good Luck with it
> Alan G3NYK
>
> ----- Original Message -----
> From: "Rob, M0DTS"<rob@m0dts.co.uk>
> To:<rsgb_lf_group@blacksheep.org>
> 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 th=
e
> 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 Sch=E4fer 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 Sch=E4fer 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
>>>
>>
>
>
>