I have a very old Skymap panel mounted on the left side, & a Dittel radio centre panel.
On some? frequencies I get a regular pulsing interference "transmission?" from the GPS which cuts the intercom (as does any signal being received) & also makes transmitting impossible. The GPS signal / map display is normal.
If I switch off the GPS it solves the problem.
The GPS antenna is suction cup attached to the left side of the screen & excess cable coiled as far away as pos to the left to avoid the radio.
Can this be fixed or is it the freq I am using 129.9 ?
GPS Radio interference
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Trevor Harvey
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Frank Parker
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There have been reports that certain Narco radio IF stages can interfere with the local oscillator in older GPS units, and also reports of re-radiation from ELT units, which cause the GPS to lose signal. I suppose that it is possible that the reverse may happen, and the local oscillator in the GPS is interferening with your comm radio. If this is the case, it should not happen on all frequencies. Easy to check!
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Trevor Harvey
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Andrew Leak
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I had a similar prob with a Skymap until I re-routed the radio supply to a separate bus-bar from the GPS. I still got a slight pulse through but managed to get rid of it with 150mhz ferrite clip ons to the GPS and Radio. It may be worth checking your radio aerial as base plates or indeed the insulators do play up when it's been a few years since new!
Andrew
Andrew
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Trevor Harvey
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es
Thanks for the replies. I'll have a look at the Bendix site, may be worth a phone call also.
During the winter of 97/98 when designing stuff for Skyforce a similar problem occurred with VHF/UHF tranceivers mounted in RAF Hawk trainers that had been equipped with Skymap2 GPS units.
After a couple of frustrating days working on a live jet at Manston we decamped to RAF Leeming with our cars full of test gear to where most of the afflicted fleet were based. Unusually however the Red Arrows fleets weren’t affected but the Leeming and Mona squadrons were.
After a day’s work we had a theory but couldn’t square it with the unaffected Red Arrows fleet until I called into Cranwell on the way home and spotted the essential difference.
In the case of the training fleet important earth bonding straps which were normally attached between the floating instrument panel and coaming panels were not refitted after servicing resulting in the panels themselves, to which the Skymap was attached in place of the gun sight, becoming RF “hot” and acting as very effective antennas in their own right.
As a result quite small amounts of stray RF from the Skymap circuit boards was being coupled to the floating panels by capacative coupling and thus re-radiated into the fuselage mounted slot antennas which fed their signals straight into the receivers.
The problem was worsened by the fact the installations had to be “hole” free, that is we weren’t allowed to drill any attachment holes in the Queen’s aeroplanes so the antenna moulding was suckered on to the front of the coaming near the windshield base. It too inductively coupled some RF into the coaming.
As you have described they primarily affected the VHF frequencies though they were also detectable further up the spectrum.
Things were so bad that in some instances our test equipment was picking up significant levels of RF current across the main wing panels particularly on 117.65 which was due to interference generated by the 3.2768 Mhz master oscillator in the Skymap identifiable by its regular one second rasps.
The Cranwell fleet’s panels were however bonded to the main airframe by the earth straps and so did not exhibit the same problems. They also properly mounted the antenna to better cope with the g excursions.
As a result all the bonding straps were refitted to the fleet and the problem went away.
At the same time some of the early Skymaps were stripped and reassembled to ensure the sprayed on conductive coating on the inside of the mouldings was effectively internally grounded. In a number of cases this was found to be compounding the problem.
Two possibilities therefore for you to consider:
Firstly ensure the instrument panel is effectively grounded to the rest of the airframe and that antenna grounds are effective.
Secondly if the unit has been opened up at some time for battery replacement or repair to ensure the internal screening is still effective.
There is a significant but simple means of RF jointing the major moulding to each other which may have been omitted or damaged.
I had a list of frequencies that were most likely to be affected based on knowledge of the Skymap master oscillator frequency and the local oscillator and IF frequencies within the affected comms equipment. This might still be in the archive but I am not surprised by your quoted example as in the above example the 36th overtone was the problem.
If however yours is the old “black and white” Skymap original design then they are rather difficult to repair and prevent causing RFI problems.
Best of luck!
After a couple of frustrating days working on a live jet at Manston we decamped to RAF Leeming with our cars full of test gear to where most of the afflicted fleet were based. Unusually however the Red Arrows fleets weren’t affected but the Leeming and Mona squadrons were.
After a day’s work we had a theory but couldn’t square it with the unaffected Red Arrows fleet until I called into Cranwell on the way home and spotted the essential difference.
In the case of the training fleet important earth bonding straps which were normally attached between the floating instrument panel and coaming panels were not refitted after servicing resulting in the panels themselves, to which the Skymap was attached in place of the gun sight, becoming RF “hot” and acting as very effective antennas in their own right.
As a result quite small amounts of stray RF from the Skymap circuit boards was being coupled to the floating panels by capacative coupling and thus re-radiated into the fuselage mounted slot antennas which fed their signals straight into the receivers.
The problem was worsened by the fact the installations had to be “hole” free, that is we weren’t allowed to drill any attachment holes in the Queen’s aeroplanes so the antenna moulding was suckered on to the front of the coaming near the windshield base. It too inductively coupled some RF into the coaming.
As you have described they primarily affected the VHF frequencies though they were also detectable further up the spectrum.
Things were so bad that in some instances our test equipment was picking up significant levels of RF current across the main wing panels particularly on 117.65 which was due to interference generated by the 3.2768 Mhz master oscillator in the Skymap identifiable by its regular one second rasps.
The Cranwell fleet’s panels were however bonded to the main airframe by the earth straps and so did not exhibit the same problems. They also properly mounted the antenna to better cope with the g excursions.
As a result all the bonding straps were refitted to the fleet and the problem went away.
At the same time some of the early Skymaps were stripped and reassembled to ensure the sprayed on conductive coating on the inside of the mouldings was effectively internally grounded. In a number of cases this was found to be compounding the problem.
Two possibilities therefore for you to consider:
Firstly ensure the instrument panel is effectively grounded to the rest of the airframe and that antenna grounds are effective.
Secondly if the unit has been opened up at some time for battery replacement or repair to ensure the internal screening is still effective.
There is a significant but simple means of RF jointing the major moulding to each other which may have been omitted or damaged.
I had a list of frequencies that were most likely to be affected based on knowledge of the Skymap master oscillator frequency and the local oscillator and IF frequencies within the affected comms equipment. This might still be in the archive but I am not surprised by your quoted example as in the above example the 36th overtone was the problem.
If however yours is the old “black and white” Skymap original design then they are rather difficult to repair and prevent causing RFI problems.
Best of luck!