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Garmin SL-30 - how to decrease output power?

tmo wrote:

Could it be the supply side going too low voltage

Maybe not even continuously too low but merely generating a short brownout on the power bus.

LKBU (near Prague), Czech Republic

Yes, could well be supply loading. Easy to check with a scope on the supply though.

The resetting issue can be tracked down by putting capacitors on various wires going into the fuel instrument, etc.

Administrator
Shoreham EGKA, United Kingdom

Could it be the supply side going too low voltage for the intercom / fuel computer to go haywire? But I guess lowering the output power would help there as well.

tmo
EPKP - Kraków, Poland

One good way to see where the offending RF is getting radiated is to disconnect the antenna and put a 50 ohm terminator there.

The connector may be different from the above pic.

You can buy these on Ebay.

This is how I discovered that the VHF 121.20 etc affecting the GPS was due to radiation from cables, not from the antenna on the roof. But I could not get to the cables as they were inside the centre stack so that was a big job. I solved it with notch filters.

These terminators can’t dissipate the 8W for more than a few seconds; you need a bigger one for that e.g.

but it will work for long enough.

However I agree with Robin in that the cause is most likely radiation from the antenna. One needs to work on immunity of the affected wiring.

Administrator
Shoreham EGKA, United Kingdom

I don’t think it is the coax cable that is radiating, the field from antenna is much stronger. Easy check install dummyload at antenna still using cable, if problems still excist cable has influence, if not you have immunity issue

Robin_253, inserting an inline attenuator is indeed a very good and non-destructive idea. Not so with RG-58: I suspect RG-58 would have a higher level of parasitic RF radiation than RG-400. By the way, while RG-58 does indeed have an attenuation of 16 dB/100 m at 100 MHz, for RG-400 it’s 4.5 db/100 ft, which is about 14.7 dB/100 m – the difference is really marginal, that’s why I mentioned LMR-200, which is indeed a lot better in that aspect (about 3 dB/100 ft ≈ 10 dB/100 m).

LKBU (near Prague), Czech Republic

Dear Przemek,

The problem is that many have poor knowledge on RF. I doubt that lower the power will prevent the issue. You far more likely have an immunity issue, e.g. which you should solve at the “unwanted receiver” side of your fuel system and intercom. When using good installation techniques this shouldn’t be an issue.

If you persist to check this reduce in power output will work, you can easily check a couple of things. The change from 8 to 6 Watt, is about 1.2 dB (which is a realy low value). In this case you could use an inline attenuator of 1.2 dB, or a more easy to find 2 or 3 dB. With this you could check if your system would work. Your receive would also be attuated, but still within limits (regulations say an upto 3 dB loss is acceptable). Another way to meet this, is using longer coax cable. The problem at your aircraft is just the field strength of the transmitted power, which causes issues, not the radiation of the cable. So while the RG-400 is a good cable, and I usually use it a lot, it doesn’t solve your issue. The RG-400 cable has a better shielding, but also has less attenuation. RG-58 has an attunation of about 16dB/100m or 0.16dB/m For RG-400 it is about 4,5 dB/100m. So if you need an 1.2dB attenuation, you could make your cable from RG-58 and make it 1.2/0.16 = 7.5 meter longer. Keep excess coax at the antenna end, coil it in X loops, as permant solution. If this is the solution, you would stay within legal limits for receiver sensitivity and power output.

To test it you could also disconnect your Icom and use and handheld using the same coax, usually hand helds have much lower output, so you can make a quick to see if that would help or not.

When you have an hand held radio, and multiple attenuators or a switched attenuator (beware of power ratings) You could give lots of attenuations, such that you can use the hand held antenna to probe around, and see which parts of the system are sensitive.

You most likely would end up, needing to improve immunitiy from the equipment, by shielding that equipment, and it’s wiring. For audio, use shielded wiring, don’t make ground loops, for fuel system, also use shielded wiring. What kind of equipment do you need, don’t hesitate to contact Jesse at [email protected]

The problem is that you will never know how marginal the issue is.

I would start with the fuel computer and re-do the cabling with a shielded cable. Most people don’t use shielded cable there. I used to get a 7% error due to a pickup of (presumably) ignition harness emissions – here. Make sure the shield is continuous, all the way from the transducer to the instrument case. Also any wires going out of the instrument (e.g. to a GPS) need to be shielded, and again continuously to the GPS case. Ground loops are relevant only if there is current flowing through the ground, which won’t be the case if the wiring is done right.

If you like, email me the wiring diagrams and I will take a look.

Administrator
Shoreham EGKA, United Kingdom

Hi. I have new RG-400, high quality connectors, metal ground plane under antena and low SWR.
Also no ground loops in installation (is new).
Problem is that when I trasmit my intercom loops and/or fuel flow computer reset.
This is small, wooden aircraft and it’s just too close and too strong, that’s why I’m looking way to decrease to 6W with I’m sure decrease level of interferences.

EPKZ

The antenna will definitely need “something” in the bottom – I guess a ground plane is the usual way because you get less radiation into the cockpit than with a dipole.

Re the Q about crimping, I don’t know which is better. Here is an old thread on crimping but it doesn’t cover that. Crimping relies on creating a gas-tight joint and I would think you get a better opportunity to create that with a stranded wire.

Administrator
Shoreham EGKA, United Kingdom
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