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VHF antenna problem

Aug 21, 2020
23
Catalina 275 Coffs Harbour
My VHF radio has good reception, but weak transmission (based on multiple radio checks with our local marine rescue). I checked the pl-259 connector with a multimeter and discovered there is a short between the inner pin and outer connector. Question 1 - is this likely to be the cause of the weak transmission?

Thinking it must be a bad solder job within the pl-259 connector, I decided to replace it with a new one. However, on removal of the plug I discovered the short is still there (zero resistance between the shielding and inner wire). Any suggestions on locating the short (most likely spot)? I was thinking a trip up the mast to check the antenna is probably the best option but I don't really know what to look for. Any advice would be appreciated.
 
Jan 25, 2011
2,223
S2 11.0A Anacortes, WA
What do mean by a short? What scale on the VOM did you use? My guess is you are measuring through the coil on the antenna and you should get a few ohms at most on the X1 scale. That would be normal. Do you have a disconnect at the mast base? You can take a trip up the mast and disconnect the antenna and the the coax should measure totally open..you can also obtain an antenna with some coax and test your radio with it.
 
Oct 22, 2014
16,100
CAL 35 Cruiser moored EVERETT WA
That can be a difficult thing to isolate. I like the path you took to examine the connections. It is a logical place for a wire to be out of place. There is another pl-259 connection from the coax to the antenna.

Did you ever have a time when the transmission worked? If so, then you need to look for damage along the coax. If it worked and now it doesn’t then something has happened. Perhaps a screw has penetrated the coax. Maybe the coax bends over an edge and there is chafe showing the damaged point. Alternatively it could be the coil that tunes the antenna or the fittings that plugs into the antenna.

It is a case of inspection and elimination as you explore along the path from the radio to the antenna.
 
Sep 25, 2008
6,315
Alden 50 Sarasota, Florida
Mark indicated the usual reason - most marine VHF antennas have a loading coil so when you look at the coax while it is connected to the antenna, it appears to be a short. The coil conducts DC but not RF so your observation of a short is misleading.

The only reasons a PL259 connector goes bad are 1. Water/moisture from a poor seal or 2. Stress failure from vibration. Check all connections.

And don’t ignore the possibility of a bad RF amp in the radio failing which can reduce output power. Check with a SWR/power meter to determine if the transmitter is producing rated power output into a dummy load (not the antenna).
 
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Jun 11, 2004
1,220
Oday 31 Redondo Beach
Check with a SWR/power meter to determine if the transmitter is producing rated power output into a dummy load (not the antenna).
Can you suggest such a meter that would be useful but won't break the bank for someone who would not be using it that often?
Thanks.
 
Jul 23, 2009
496
Beneteau 31 Oceanis Grand Lake, Oklahoma
Check out this thread.
 
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CarlN

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Jan 4, 2009
577
Ketch 55 Bristol, RI
How old is this stuff? If over 10 years it's probably easier to just replace than diagnose. Salt air eventually gets to the PL259 connectors. Ideally, replace the antenna, connectors and coax. If it's too hard to pull new coax then put on new connectors (assuming there's enough cable to cut it back 3" at each end). If you are not expert at soldering use the Shakespeare "Gold" connectors. Just about foolproof and excellent RF performance. Pack the connector with dielectric grease and then wrap with silicone "rescue" tape. My favorite dielectric grease is NO-OX-ID. Used by utility linemen for decades. Won't wash out. Amazon.com: NO-OX-ID A-Special- Electrical Contact Grease- Keeps Metals Free of Rust and Corrosion- Part# 10203: Home Audio & Theater
 
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Aug 21, 2020
23
Catalina 275 Coffs Harbour
Thanks everyone for all the helpful information. It sounds like I might have interpreted the low resistance of the loading coil as a short circuit. I was not aware that even existed, but I'm measuring 2ohms which concurs with Mark's advice.

In terms of age, the boat is 5 years old, but I just replaced the radio with a new one that has an AIS receiver. The radio check after replacing the radio alerted me to the issue and I initially thought it was the new radio. I then reconnected the original radio and got the same report - weak signal, 3 out of 5. Also, the power source is good, I measured the voltage drop while transmitting (at 25watts) and it went from 12.7V down to 12.3V.

I don't have the equipment to test SWR, and I'm starting to feel a bit out of my depth, so maybe it's time to start looking for a technician.
 
Feb 17, 2006
5,102
Lancer 27PS MCB Camp Pendleton KF6BL
I agree with some of the mentioned possible problems. If you use a multi-meter on an antenna with a coil you will get a short condition. Not to be mistaken with a real short. If that were the case, even a tiny wire touching something that has the reverse polarity will cause the radio to completely shut down.

So this leads to the antenna. It too is possibly not working well. However, it should be noted that if there was an issue there then it would affect your RX as well as you TX.

Next is the cable. This is probably the more logical point. The reason I say that is even a paper clip plugged into the antenna jack will receive something. Remember the cloths hanger trick on cars? No real need for a ground when receiving. But! if the ground on the cable is not making a good connection then it WILL affect your transmit. Bad ground mean poor return means high SWR means low output power, if any output power.

So, not saying 100% your cable, but that I the first place I would look. Specifically your connectors to make sure your grounds are clean and intact with the PL-259s. On both ends.

JMHO
 

CarlN

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Jan 4, 2009
577
Ketch 55 Bristol, RI
I don't have the equipment to test SWR, and I'm starting to feel a bit out of my depth, so maybe it's time to start looking for a technician.
A technician able to measure SWR will cost you far more than replacing the antenna and connectors. A "3 out of 5" is almost certainly corrosion in a connector that is adding resistance and robbing signal. A problem with the radio would likely show no measurement.
 
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Jan 25, 2011
2,223
S2 11.0A Anacortes, WA
Is there another boat you can move the radio to? Just gin up some clip leads for power..And/or talk to your radio check destination from another boat and compare..John’s idea of a cheap VSWR meter is a good one. It should have instructions. Otherwise google is your friend and we could get you through it.
 
Oct 29, 2012
289
Catalina 30 TRBS MkII Milwaukee
What size is the coax installed going up to the mast head? Too often the length from the radio transmitter, through the boat and the up the mast can reach 100'. If the coax installed is not sized to consider the length it can cause tremendous SWR and greatly reduce signal strength. This will also lead to weakening, or causing failure to the final amplifier output.
CABLE:20'40'60'80'
RG-58CU-1.2dB-2.4dB-3.6dB-4.9dB
RG-8X-0.9dB-1.8dB-2.7dB-3.6dB
RG-213-0.5dB-1.0dB-1.6dB-2.1dB
 
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Sep 25, 2008
6,315
Alden 50 Sarasota, Florida
What size is the coax installed going up to the mast head? Too often the length from the radio transmitter, through the boat and the up the mast can reach 100'. If the coax installed is not sized to consider the length it can cause tremendous SWR and greatly reduce signal strength. This will also lead to weakening, or causing failure to the final amplifier output.
CABLE:20'40'60'80'
RG-58CU-1.2dB-2.4dB-3.6dB-4.9dB
RG-8X-0.9dB-1.8dB-2.7dB-3.6dB
RG-213-0.5dB-1.0dB-1.6dB-2.1dB
Just for clarity, longer runs of coax can effectively decrease apparent SWR at the meter unless the meter is inserted at the antenna end which is usually impracticable. When placed at the junction of the coax at the back of a radio, longer runs and high loss coax actually reduces apparent SWR.
The 2.8dB loss difference between 80ft of 58 vs 213 is effectively a 50% reduction in radiated power with a similar attenuation of received signals.
 
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Ward H

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Nov 7, 2011
3,128
Catalina 30 Mk II Barnegat, NJ
You mentioned you checked the PL259 connector. How many do you have? On my C30 I have 4. One at nav station where radio is installed, 2 at base of mast with female - female connector and one at the antenna.
When I replaced my radio a couple of years ago I had the same symptoms. Since I had installed two of the crimp connectors, nav station and 1 at the base of mast, I figured I did a poor install and changed them one at a time. Finally I changed the 2nd one at the mast base. This one had been installed by the yard when I first put the mast up after buying the boat.
That was the bad one and I'm glad it was or I would have had to go up the mast.

Moral of that story is connectors are relatively cheap. Just go down the line and replace. I used crimp connectors with plans to go back and install soldered connectors.

BTW, my C30 has internal halyards. I pulled the halyard exit block out to get access to the mast base connectors.
 
Aug 21, 2020
23
Catalina 275 Coffs Harbour
Good news - I found the problem and have fixed it. The problem was the PL259 connector that attaches to the back of the radio. Whoever fitted that connector messed up as there was a direct connection between the shielding and inner core. I resoldered a new one and received a 5 out of 5 radio check. The reception was also noticeably louder and clearer.

You may well ask why I didn't realise this earlier (since that was my original suspicion), and that would be a good question. It all stems back to my original incorrect assumption that there should be NO electrical connection between the shielding and the core. This came from a video demonstrating how to solder a new PL259 connector, suggesting you should test your work by using the continuity test on the multimeter to check for an open circuit between the centre pin and the outside of the connector. Obviously, that person was testing a cable that wasn't connected to an antenna. Now that I know about the loading coil, I should have instead been measuring the resistance. When
I cut the suspect connector off the cable I made the real error - instead of retesting the plug, I repeated the continuity test on the exposed wire. Because it returned the same result I assumed the plug was not the problem, and that another issue existed somewhere up the cable.

With the better understanding I know have (thanks to you all), I now see that measuring the resistance in the first place would have revealed that yes there was a problem, and re-measuring it on either the severed plug or the exposed cable would have identified the problem immediately. I am very sorry it took so long to reach this conclusion and am extremely grateful for all of the suggestions offered above. As well as fixing the problem, I have learned a lot in the last couple of days!