[Antennaware] Antennaware Digest, Vol 117, Issue 1

Tue Feb 5 10:45:55 EST 2019

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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> >    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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