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High VWSR on ELT test

bob888

Well Known Member
When I test my ELT I get an error (series of beeps) indicating high VWSR. The unit is ACK E04. Anyone able to explain the error and what to do about it? Manual doesn’t really help.
 
There’s a problem with coax, coax connectors, or antenna. If you can substitute a known good piece of coax and re-test that may help narrow it down.
 
Has it been working properly before or is this a first-time system check?
On a first time, there may be some quite simple reason for the failure. Otherwise, something must have degraded or got broken over time.
Disconnect the coax cable on both ends and check for good insulation between center pin and screen (multimeter), then check conductance from pin to pin and from outer sleeve to outer sleeve. And check the coax cable for sharp turns, bends, kinks or other damage. Once this has been found good, check for proper ground connection at the antenna mount. If that doesn't help you to find the root cause, some photos of the installation might help. (Connectors, ELT mount, antenna mount, etc.)
If you have a separate antenna at hand, remove the ELT, connect the antenna directly to the ELT and re-perform the test outside your installation. Had a faulty ELT on a first-time installation myself and this test helped to identify it. Vendor was most supporting and replaced it with a good one.
 
I had a similar problem with known-good coax and connectors on my first test. I initially had the antenna mounted on a flange off the longeron in the baggage compartment; the antenna gently bent along the inside of aft canopy, like I've seen many other RVs do. RF-wise, that was a bad location, and I bit the bullet and mounted it outside on the aft fuse about 3" from the VS and problem solved.

I didn't spend much time fiddling with the original install since I had my AW inspection coming up and didn't want the DAR to test it and fail me! (I was a bit paranoid then)

A good check would to mount the antenna on a 2' square scrap aluminum, mount somewhere free of nearby conductors, and run the cable out of the plane from the ELT and run the self test. That would help to differentiate between an install location issue and a cabling issue.
 
VSWR

A more reliable solution is to purchase a BNC 50 ohm dummy load (available through amazon or your favorite electronics component house) and a back to back adaptor to connect directly to your ELT. If you still get a VSWR beeping indication then there is likely a problem inside the ELT and it needs to be returned to the manufacturer for servicing.

KT
 
Antenna cable length

If this is a new installation.
The installation manual does not, in most cases, say anything about the length
of the antenna cable. It is understood that the avionics shop knows how to
select the correct length of the antenna cable.
The length of the antenna cable should match the ELT-frequency.

Good luck
 
That’s not how coaxial cable works when used as a simple transmission line.

+1 on that. But depending on the design of the power output stage of the transmitter there may be a minimum loss required between the ELT output connector and the antenna to ensure stable transmitter operation ( read minimum length of coax to provide more than the minimum loss). VSWR can always be improved by either providing a better match or by inserting an attenuator pad at the ELT output connector. Typically 6 ft of RG58 provides enough loss with even a very poorly matched antenna to bring the VSWR into the stability range for most GA ELT units. The important point is to ensure that the coax connector end connected to the ELT is correctly installed.

KT
 
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Monopole antenna

I wouldn't tend to attribute any improvement to cable losses. 6 ft of RG400U would produce about 0.5 dB of loss at these frequencies which is near to nothing.
Reading the Wikipedia article about how monopole antennas work, I'd suspect the improvement observed comes from the cable adding additional RF grounding capacity to the antenna footpoint. That said, any better grounding of the footpoint (min. 1 sqft. of sheet metal) should improve the VSWR. This applies as long as the antenna is properly tuned to the TX frequency. Metal parts near to the antenna rod can heavily de-tune an antenna.
If grounding and wiring seems good, it will take an antenna analyzer to find the root cause. You might find help from a radio amateur near to you, easily to be identified by the huge antenna tower in his backyard.
 
ARTEX ELT

Connect RF coaxial cable
The BNC to BNC cable provided carries both the 121.5 MHz and 406 MHz outputs. The ME406 was certified using a 6 foot (1.8M) RG-142 (MIL-C-17) coax cable. Maximum cable loss should not exceed 1 dB.
In all cases the coax must meet or exceed RG-142 specifications.
Contact ARTEX if other lengths of coax are required for your installation.
If low loss cable is required, see Paragraph 2.7.5.1.

Good luck
 
A good example of how antenna placement is paramount to a receiver/transmitters operation.
No/inadequate ground plane = high VSWR
Your coax likely is fine.
 
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Smith Charts, VSWR and Power Stage Stability

I wouldn't tend to attribute any improvement to cable losses. 6 ft of RG400U would produce about 0.5 dB of loss at these frequencies which is near to nothing.
Reading the Wikipedia article about how monopole antennas work, I'd suspect the improvement observed comes from the cable adding additional RF grounding capacity to the antenna footpoint. That said, any better grounding of the footpoint (min. 1 sqft. of sheet metal) should improve the VSWR. This applies as long as the antenna is properly tuned to the TX frequency. Metal parts near to the antenna rod can heavily de-tune an antenna.
If grounding and wiring seems good, it will take an antenna analyzer to find the root cause. You might find help from a radio amateur near to you, easily to be identified by the huge antenna tower in his backyard.

I mentioned RG58 not RG400 - big difference. While on Wikipedia you should look up Smith Charts and RF amplifier power stage stability for a better understanding of stability margins and how the complex impedance of a coax and mismatched antenna affect the power output stage. Since most ELTs transmit on both 406 and 121.5 MHz with a simple monopole antenna it is difficult to get a good match at both frequencies (Since they are not direct harmonics) without a complex antenna matching network. An acceptable solution is to have a long enough coax cable using a coax type that has sufficient loss to not result in damage or instability in the output stage. Typically the output stage will have a microstrip directional coupler that can provide a go no go indication of the VSWR to ensure the returned power to the transmitter stage is less than a required level. Fixing the loss to be in the coax to be more than a minimum level reduces the requirements on the antenna match at both frequencies.

KT
 
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