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

[Burt I. Weiner]

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