We have a BK 97A fitted to a Piper L18C and are baffled by radio transmissions fading after a few seconds, the radio receives perfectly.
Following ground tests we have found that as the engine revs increase the transmission fades sooner.
Does anyone have any experience with this or can they recommend a radio specialist in the White Waltham area.
Many thanks, Robert
Radio Fades During Transmission
It would be worth checking the aerial lead and the match between the set and the antenna. Co_ax plugs and terminations are notorious for alowing entry of moisture and the set may not be seeing the 50 ohm impedence it wants to see. Some sets if loaded with a significantly different impedence will back off the power output to preserve the output transistors or they may go into thermal shutdown. If you can lay hands on a power meter check the forward and reflected power on the feeder.
I agree with the previous reply.
One note though: I don't think it's a power meter you need - it's a SWR (Standing Wave Ratio) meter. This measures the efficiency of the antenna in terms of the amount of power reflected back down the feed to the radio from the antenna. They're good for checking the integrity of the antenna leads as well depending where you put the meter: between the radio and the lead or the lead and the antenna.
The SWR meter is generally used to tune antennas where they are either adjustable for length, or need to be 'matched' by another box of tricks. As far as I know this doesn't normally apply to aircraft radio antennas, these being the correct length in the first place.
As to how to use the meter, ask whoever you get it from to explain / demonstrate it as they vary a bit. It is a 2 step process though, setting the meter and then taking the reading.
Is an L18C a steel tube fuselage? Assuming it is and the antenna is somewhere on the fuselage then is there sufficient ground plane?
Basically a ground plane is an electro-magnetically reflective surface around the base of the antenna, and the screening braid of the antenna lead should be connected to this. On an ally aircraft (Cessna, Vans etc) then the fuselage is all ground plane. On wooden aircraft (VP1 etc) I stuck a layer of cooking foil to the underside of the panel where the antenna is mounted. Double sided carpet tape did the sticking job excellently.
Not actually worked with radios / antennas etc on a steel tube fuselage (yet). Maybe somebody with this experience can contribute.
One note though: I don't think it's a power meter you need - it's a SWR (Standing Wave Ratio) meter. This measures the efficiency of the antenna in terms of the amount of power reflected back down the feed to the radio from the antenna. They're good for checking the integrity of the antenna leads as well depending where you put the meter: between the radio and the lead or the lead and the antenna.
The SWR meter is generally used to tune antennas where they are either adjustable for length, or need to be 'matched' by another box of tricks. As far as I know this doesn't normally apply to aircraft radio antennas, these being the correct length in the first place.
As to how to use the meter, ask whoever you get it from to explain / demonstrate it as they vary a bit. It is a 2 step process though, setting the meter and then taking the reading.
Is an L18C a steel tube fuselage? Assuming it is and the antenna is somewhere on the fuselage then is there sufficient ground plane?
Basically a ground plane is an electro-magnetically reflective surface around the base of the antenna, and the screening braid of the antenna lead should be connected to this. On an ally aircraft (Cessna, Vans etc) then the fuselage is all ground plane. On wooden aircraft (VP1 etc) I stuck a layer of cooking foil to the underside of the panel where the antenna is mounted. Double sided carpet tape did the sticking job excellently.
Not actually worked with radios / antennas etc on a steel tube fuselage (yet). Maybe somebody with this experience can contribute.
Rob Swain
If the good Lord had intended man to fly, He would have given him more money.
If the good Lord had intended man to fly, He would have given him more money.
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Andrew Leak
- Posts: 79
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- Location: Southampton
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Hi Robert,
Couple of things you didn't mention:
1. Has the radio, co-ax or antenna been moved before the transmission problems?
2. Have you checked your earth on the radio lead or the fuse (trip fuses tend to wear electrically with age).
3. The next thing is to check your battery, does it discharge noticeably when you transmit?
4. Do you have an alternator? Is this insulated against interference? Is the regulator working OK?
5. As it's engine related, are your mag leads shielded and earthed still?
6. Is the transmit power loss evident throughout the whole frequency range or just the lower frequencies?
Ground planing is always an interesting one as you will need one with a tube or wood and rag aircraft, although some owners would argue that their non-ground planed aircraft is ok, with just using the metal tubing.
If you need to get technical about power and wave ratios, look here:
http://continuouswave.com/whaler/reference/VHF.html
http://www.tapr.org/ve3jf.dcc97.html
hope this helps,
Andrew
Couple of things you didn't mention:
1. Has the radio, co-ax or antenna been moved before the transmission problems?
2. Have you checked your earth on the radio lead or the fuse (trip fuses tend to wear electrically with age).
3. The next thing is to check your battery, does it discharge noticeably when you transmit?
4. Do you have an alternator? Is this insulated against interference? Is the regulator working OK?
5. As it's engine related, are your mag leads shielded and earthed still?
6. Is the transmit power loss evident throughout the whole frequency range or just the lower frequencies?
Ground planing is always an interesting one as you will need one with a tube or wood and rag aircraft, although some owners would argue that their non-ground planed aircraft is ok, with just using the metal tubing.
If you need to get technical about power and wave ratios, look here:
http://continuouswave.com/whaler/reference/VHF.html
http://www.tapr.org/ve3jf.dcc97.html
hope this helps,
Andrew
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