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From: N1BUG <paul@n1bug.com>
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Message-ID: <ebf9700f-a8f7-fb34-7566-0d9501e5ee82@n1bug.com>
Date: Tue, 17 Jul 2018 17:03:01 -0400
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In-Reply-To: <5B4DE197.1000303@posteo.de>
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Subject: Re: LF: TX > RX isolation test
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Reply-To: rsgb_lf_group@blacksheep.org

Hello Stefan, thanks Andy and Roelof for input as well.

I find myself in a strange, alien world on LF. All my experience at
HF, VHF, UHF seems to be no help down here. ;-)

In hindsight, the TX > RX antenna isolation measurements I made the
other day make no sense at all to me. I now question whether the
results mean anything.

If the preamp is designed for 50 ohm input, why is a 50 ohm resistor
not a suitable 'dummy load' for receiver testing?

I can understand and accept that this antenna is not fed
appropriately and is not a good match to this type of preamp. I
built all of this in a big hurry and without much thought last
November, trying to be ready before the deep snow came. Others had
used this antenna at MF. I just took a chance that it would work at
LFG too. In fact it doesn't seem very good on either band. :-)

I have about 4 months to figure out how to make a real LF RX antenna
for this winter! I want a few more dB S/N from Europe!

The 9m vertical is in a small open area but it has trees on 3 sides.
Trees are 10m away on one side, 15m on another side, and finally
about 25m the other side. It is 12m from the TX vertical and about
22 or 23m from each of the two grounded towers (33m and 35m tall)
which support the TX vertical.

I think it's not so easy to connect the 9m vertical directly to the
preamp. It could be done by moving the preamp outside and finding a
12v battery to power it, then running coax back to the receiver 60m
away.

The reason for my interest in trying a directional antenna (K9AY,
EWE, ...) is  most of my interference comes from the southwest.
During winter there are often storms with lightning to my southwest
but not in any other direction. I also have some local man made
noise (power line problems and some industrial motor speed controls)
to the southwest, but no known noise makers to the north or east.

73,
Paul


On 07/17/2018 08:31 AM, DK7FC wrote:
> Hello Paul,
> 
> Interesting.
> I think that anyway the antenna will have a certain feed point impedance 
> to the preamp input. It can be replaced by a certain R and C, discrete 
> elements. C may be 56 pF and R may be 200 Ohm for example. But the 
> discrete elements will not generate a RX signal voltage. For the preamp 
> it should be equivalent. So you will see the system noise floor 
> (preamp+soundcard/RX etc).
> It is a bit like a dummy load for the RX. :-)
> 
> Is your 9m vertical standing on a free field or on the top of a house or 
> close to trees and other greenery? That will make a difference!
> 
> I did not yet try a 100:1 transformer and i would expect that the 
> impedance of that wire at LF is much to high (abt 20k Ohm) to be 
> converted with a normal transformer. Maybe the core material is not well 
> choosen or the internal capacity of the primary winding  is to high, so 
> you're bypassing a lot of the signal? What happens when you connect the 
> wire directly to the preamp?
> 
> 73, Stefan
> 
> 
> Am 17.07.2018 13:14, schrieb N1BUG:
>> Hi Stefan,
>>
>> On 07/16/2018 08:00 AM, DK7FC wrote:
>>    
>>> What type of preamp are you using (schematic?). For a comparison with
>>> and without antenna, you need to replace the antenna wire with a
>>> capacitor of the same C (assume 5...6 pF/m antenna wire, i.e. use 56 pF
>>> to replace a 10m vertical) connected to the preamp. If this shows as
>>> e.g. -120 dBfs it should show -105 dBfs with the antennna connected.
>>>      
>> I am not certain we are talking about the same antenna
>> configuration. This antenna is not an E probe, at least not in the
>> sense that I understand it. It's a 9m tall whip with 100:1 impedance
>> transformer at the base. The high Z primary winding connects to
>> antenna and ground. The low Z secondary goes to 60m of twisted pair
>> to the shack. At the shack there is a 2:1 transformer feeding a 50
>> ohm in/out preamp. This is the preamp I use:
>>
>> http://w1vd.com/LFMFHFpreamplifier.pdf
>>
>> I can not quite put into words what bothers me about my system but I
>> get a feeling something is wrong. I think I need to go back to the
>> beginning and try to understand noise floor / MDS at every stage of
>> the system... receiver without preamp, with preamp, etc. I am not
>> sure exactly how to proceed with this evaluation.
>>
>> Paul N1BUG