[time-nuts] Why the fuss - GPS Interference...

[Rob Kimberley]

Hi Burt,

This sounds familiar! It wasn't a guy called Rich Bailey was it? That's what
he suggested I do, but I got onto Trimble and got one of their Bullet
antennae with the 3 pole filter (Bullet III?). 

That worked for us.

Rob

-----Original Message-----
From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On
Behalf Of Burt I. Weiner
Sent: 28 September 2012 16:34
To: time-nuts at febo.com
Subject: [time-nuts] Why the fuss - GPS Interference...

I had a similar experience while working on the Seiko pager watch project
some years ago.  We were using DATUM GPS 9390xxxx GPS receivers to time the
system at each FM station transmitter site.  One particular FM site on Edom
Hill near Palm Springs, California (U.S.A.) had a problem from not only the
associated FM transmitter but also from a nearby UHF TV running a bazillion
or so Watts ERP.  We finally installed a Symerticom WWVB receiver, but that
was not as reliable as we needed.  Not because of any interference, but
because of propagation issues at 60 kHz at this mountain top site.

I called DATUM to see about filters that could be put ahead of the antenna.
The fellow I talked to lived within 50 miles of the site.  We
intellectualized for a while and finally agreed that I would pick him up in
the morning and take him to the site. Well, when I picked him up he had a
shopping bag that he put in the back seat.  We had planned on getting
something to eat before we went up the mountain so I asked, "Bringing a
snack?"  He said, "No, it's a surprise."  I also had one of the DATUM
receiver running in my car off of 12 Volts with the antenna attached to my
roof mount.

As we approached the site the receiver in my car lost all satellites.  He
looked at the receiver in the Seiko system and then went back to the car and
got the shopping bag.  He then revealed his secret - a two pound coffee can
with a few holes poked in the side near the bottom.  We went up on the roof,
disconnected the DATUM's antenna, and mounted it inside the coffee can.  He
told me that you have to be sure the antenna and the coffee can are
electrically (RF
wise) connected together.  He then placed the coffee can on the roof
orienting it so that it would have the best view of GPS satellites as
possible given the site location and "Coffee Can Aperture" .  By the time we
got back in the building the receiver was tracking four GPS birds and a
short time later was happily doing its thing.

What he did was to use the coffee can as a waveguide beyond cutoff
attenuator.  Not really as an attenuator, but as a high pass filter.  It did
attenuate the FM band signal quite a bit and attenuated the UHF TV signal
sufficiently so that it was no longer a problem for the system.  A few says
later I went back to the site and installed the "Hi Pass Filter" in a large
upside down bottle.  This ran reliably for several years until the Seiko
project came to an end.

I have since done this same trick at a few transmitter sites on Mt. 
Wilson, which overlooks Los Angeles and is home to most of the Southern
California FM and (now) DTV transmitters.

An aside:  When Seiko ended the project they wholesaled a lot of the
non-proprietary equipment out.  I purchased four of the DATUM Time and
Frequency receivers with Y2K updates.  One I gave to a "FMT-Nuts" 
buddy, one went to a buddy who runs a Metrology Lab and the other two are
running in my shoppe.  See: http://www.k5cm.com/k6OQK%20FMT%20NEW.htm

Burt, K6OQK


>Tom,
>
>We had a similar problem at a BBC site when I was selling Datum in the UK.
>We managed to get round the problem with a better antenna. The third 
>harmonic of the UHF wasn't slap bang on L1 but close enough with a 
>basic GPS antenna to kill GPS.
>
>Rob
>
>-----Original Message-----
>
>Behalf Of Tom Miller
>Sent: 27 September 2012 18:44
>Subject: Re: [time-nuts] Why the fuss?
>
>Under other issues, I have one where GPS could not be used. It was at a 
>UHF TV station where the third harmonic fell right in the L1 band. A 
>220,000 watt UHF transmitter driving a gain antenna for 5 MW EIRP will 
>always produce some third harmonic near the antenna. There was no 
>access to GPS within 1 km of the site.
>
>They were using the WWVB signal as the time and frequency reference.
>Luckily, the conversion the DTV moved them to a new channel and now 
>they can use the GPS.
>
>Tom

Burt I. Weiner Associates
Broadcast Technical Services
Glendale, California  U.S.A.
biwa at att.net
www.biwa.cc
K6OQK 


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