[FoxHunt] Old Homing In Article

Tue Mar 9 11:01:01 EST 2010

 Bob S. : At the risk of getting "really technical" here, there is some
merit to Joe Moell's idea that reverse-bias is worthwhile, but I think it's
value is a bit over-rated. That said, my own design ( adapted slightly from
his design ) does employ reverse biasing, ( using self-bias generated with
resistors ) of about 1 volt, to reduce signal leakage. I later learned it is
not really so important, partly because of my choice of PIN diodes... I use
HSMP-3890 diodes, ( surface mount ) which are low-power diodes that require
very little DC to "turn-on"... and have less capacitance when turned off,
even with zero reverse-bias.

 On the FAQ page of Joe's website, he cites the fact that his choice for PIN
diodes ( HP 5082-3080 which are obsolete now ) go from 1.1 to 0.4 pF reverse
capacitance for 0 to -4 V reverse bias. That's a 3:1 change, which would
yield a 3:1 change in the level of the leakage signal voltage, = 10 db
improved isolation. 

 At 2 meters, 1.1 pF = almost 1 Kohm of Xc, so with 2 diodes in series, ( in
the signal path ) total Xc = 2.0 Kohm. The signal then terminates into a 50
ohm output node, so it forms a voltage divider of roughly 50/2000, which =
about -32 dB. That is good enough for good DF results, but it probably isn't
too far from the limit ( maybe - 20 dB ) before adverse effects start to
become obvious... adding another 10 db of isolation ( with -4V bias ) would
improve it ( roughly ) 10 more dB, to -42 dB isolation.

 The real benefit of Joe's design would be much more obvious when that
antenna is used at higher bands, 450 and above, where the diode Xc is
significantly lower, therefore the RF ( off ) isolation = significantly
less. ( around -22 dB @ 450 with no reverse bias ) 

 In any event, PIN diodes are definitely superior ( in every case above 50
MHz ) to ordinary signal diodes...they are very much "off" even with zero
reverse-bias, compared to signal diodes.

 My own stuff uses HSMP-3890 SMT PIN diodes, which have reverse C of only
0.5 pF ( worst case ) at zero reverse bias voltage. To cut Xc in half ( with
these diodes ) would require reverse bias of 16 volts, and the measly 1.0
volt ( reverse bias ) that my own design does provide only reduces the Xc by
10%... hardly worth the effort. I haven't bothered to change the design ( to
eliminate the DC bias ) but I can see now it is pretty useless. ( I have
verified this by experiments also )

----------------------------------------------

 Beyond that topic, "floating" an antenna element ( antenna load Z = open
circuit ) is not the same as completely removing it from the area near the
DF antenna... it may be "floating" ( ungrounded ) but it is still a
conductive piece of metal, exposed to the same wavefronts as the other
elements... and it is "close enough" to the other elements ( distance
expressed in wavelengths ) so that it WILL interact "parasitically" with
them, to some degree. ( just as reflectors and directors on Yagis are not
really "connected" to anything... but their mere presence affects the
behavior of the nearby driven element ) The element separation can't be too
much greater than 0.3 wavelengths on a Doppler antenna, ( it causes
problems, explanation = complex  ) so there is no "trick" way to mitigate
this problem... 

 I've done a few other things different from Joe's basic design, ( small
improvements ) but his idea of switching both ends of each feedline is VERY
worthwhile... especially if the antenna will be used on different bands. (
e.g. 150 and 450 MHz ) His design can easily accommodate that much frequency
change, but others cannot. Thanks Joe... it works great.

Bob S.

-----Original Message-----
From: foxhunt-bounces at mailman.qth.net
[mailto:foxhunt-bounces at mailman.qth.net] On Behalf Of Charles Suprin
Sent: Monday, March 08, 2010 5:57 PM
To: Radio Direction Finding
Subject: Re: [FoxHunt] Old Homing In Article

Bob,

On that page he cites the article as an explanation of the need for 
biasing both ends.

Charles

On 3/8/2010 3:30 PM, R. Simmons wrote:
> It appears the article is on his website :
>
> http://www.homingin.com/newdopant.html
>
> He claims 20-30% BW but really it is much greater. ( if you disregard loss
> of RF sensitivity ) The PicoDopp MiniAntenna "design" frequency is around
1
> GHz, but I have used it at 146 MHz, others have used it even lower. ( but
a
> fairly strong signal is required to compensate for small antenna size )
> That = several octaves of span.
>
> Bob S.
>
>
> -----Original Message-----
> From: foxhunt-bounces at mailman.qth.net
> [mailto:foxhunt-bounces at mailman.qth.net] On Behalf Of Charles Suprin
> Sent: Monday, March 08, 2010 11:11 AM
> To: foxhunt at mailman.qth.net
> Subject: [FoxHunt] Old Homing In Article
>
> Hello Foxes and Hunters,
>
> I am looking for a copy of the Homing In article from April 1995.  It
> contains information on design decisions for k0ov's new Roanoake
> antenna system. Joe points the user to it at
> <http://homingin.com/newdopant.html>  and Bob Simmons ( WB6EYV )
> alludes to the content at
> <http://www.silcom.com/~pelican2/PicoDopp/PICO_MORE.html>. It appears
> to deal with the need for maintaining the impedance at the antenna's
> for repeatable performance.
>
> Any insight would be appreciated.
>
> Charles
> AA1VS
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