[Antennaware] Antennaware Digest, Vol 117, Issue 1

Tue Feb 5 11:04:43 EST 2019

Isn't this list intended primarily for antenna modeling topics?  I think 
you're much more likely to get a responsive answer on Topband or Towertalk.

73, Pete N4ZR
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On 2/5/2019 10:45 AM, Joe via Antennaware wrote:
> Hey Ward, et al,
> This is not related to the original topic but you mentioned BOG antennas and I have a question.Is installing a BOG near the ground system of an Inverted-L dangerous for a receiver? Separation between them would be about 1/8 wavelength, perhaps less, from the nearest radial.
> In my situation, there would be radials and the BOG(s) in the same body of water. I have no clue how much RF current and voltage would be in there.
> Thanks, Joe, K8MP
>
> -----Original Message-----
> From: Ward Silver <hwardsil at gmail.com>
> Cc: antennaware <antennaware at contesting.com>
> Sent: Tue, Feb 5, 2019 09:57 AM
> Subject: Re: [Antennaware] Antennaware Digest, Vol 117, Issue 1
>
>
> I would suggest that you contact K9AY directly about the impedance mismatch
> - k9ay at k9ay.com
>
>  From my experience and in reviewing a lot of literature, unless the loop is
> *really* small, preamplifier noise figure is not significant on the lower
> HF and MF bands.  I suppose it might be an issue for desktop rotatable
> loops or "Beverage on Ground".
>
> 73, Ward N0AX
>
> On Tue, Feb 5, 2019 at 5:43 AM Andrew Ikin <andrew.ikin at btopenworld.com>
> wrote:
>
>> Hello Ward,
>>
>> I think I need to rephrase my email. What I am trying to say is, a small
>> terminated antenna e.g. a Flag or a Waller Flag is noise limited by the
>> termination resistor. This is because the resistor has a 3dB noise figure.
>> The antenna loss or low radiation resistance doesn't matter so much as this
>> loss can be made up with an LNA. The issue is made worse because most
>> practical
>> designs have the matching transformer z that is the same as the termination
>> resistor. Now if we use amplifier with high input z of say 30k Ohms; using
>> chemandy.com Return loss and mismatch loss calculator
>> the resistor noise is miss-matched to the amp. by approx. 10dB.
>>
>> Thus the antenna noise floor should be lower. However, I am assuming that
>> the LNA has negligible noise when terminated with the antenna. This could
>> be
>> a design issue to get the noise figure down to less that 0.5dB!!!
>>
>> The example I gave using a high z amp. with the K9AY suggests that there is
>> no loss in gain by using increasing the input z.
>>
>> 73
>>
>> Andrew  G8LUG
>>
>> -----Original Message-----
>> From: Ward Silver
>> Sent: Monday, February 4, 2019 7:50 PM
>> To: antennaware at contesting.com
>> Subject: Re: [Antennaware] Antennaware Digest, Vol 117, Issue 1
>>
>> A flux-coupled transformer with separate windings provides galvanic
>> isolation between the feed line shield's outer surface and the loop.
>> Coupling from the loop to the feed line can upset the pattern and couple
>> noise on the feed line into the antenna.
>>
>> It is also a good idea to add a choke of several thousand ohms on the feed
>> line at the antenna to prevent common-mode current on the outside of the
>> feed line shield from getting into the cable and becoming differential-mode
>> signals.  Same concern at the receiving end although most receivers are
>> well-shielded.
>>
>> 73, Ward N0AX
>>
>> On Mon, Feb 4, 2019 at 11:00 AM <antennaware-request at contesting.com>
>> wrote:
>>
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>>> Today's Topics:
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>>>      1. Fw: source impedance of resistor terminated antennas (Andrew Ikin)
>>>
>>>
>>> ----------------------------------------------------------------------
>>>
>>> Message: 1
>>> Date: Sun, 3 Feb 2019 23:04:32 -0000
>>> From: "Andrew Ikin" <andrew.ikin at btopenworld.com>
>>> To: <antennaware at contesting.com>
>>> Subject: [Antennaware] Fw: source impedance of resistor terminated
>>>          antennas
>>> Message-ID: <2E4939776E6344ECB4242B584629630C at DESKTOPCPQEP29>
>>> Content-Type: text/plain;      charset="UTF-8"
>>>
>>>
>>>
>>> From: Andrew Ikin
>>> Sent: Sunday, February 3, 2019 10:38 PM
>>> To: antennaware at contesting.com
>>> Subject: source impedance of resistor terminated antennas
>>>
>>> Is there any reason why most resistor terminated antennas like the K9AY
>>> and the Flag use a matching transformer to the Rx feed line that has the
>>> same impedance as the termination resistance. The reason for the question
>>> is, such antennas have a ideal match to the 3dB noise generated by the
>>> termination resistor and this resistor has no radiation resistance.
>> Hence,
>>> for antennas like a small size Flag, the performance could be limited by
>>> the resistor noise.
>>>
>>> However, if the antenna is connected to a high zin amp., then the
>> resistor
>>> noise should be reduced by the miss-match and if the amp. noise figure is
>>> very low, then it may be possible improve the antenna performance?
>>>
>>> Winding the clock back 20 years I noticed that the K9AY  Rx matching
>>> transformer could be replaced by a high zin  amp. with a voltage gain of
>>> 11dB. The overall gain increase compared to using 9:1 z transformer was
>>> 20dB. This increase in gain is what would be expected (11dB amp. plus 9dB
>>> by replacing the 9:1 transformer). Thus, the antenna gain compared to one
>>> without the amp. had not been degraded by using high zin amp.. However,
>>> then I didn?t consider the effect of resistor noise, but this wouldn?t be
>>> an issue with the K9AY.
>>>
>>> Is there something else I need to consider?
>>>
>>> 73
>>>
>>> Andrew Ikin
>>> G8LUG
>>>
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>>> End of Antennaware Digest, Vol 117, Issue 1
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