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Old 02-07-2016, 01:39   #1
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VHF mystery

I did a regular VHF test with a friend of mine. Both have mast top antennas at about 17 m above water. Distance was about 7 nm with some islands in between (not very high, but some trees). I could hear load and clear him even when he was sending at 1 W, but he could not here me at all even at 25 W with squelch set very low. I have a 11 years old Simrad RD68 and my friend has a 8 years old Raymarine.

Both systems have been working very well previously. My friend has installed an active antenna splitter a few years ago and we both take our masts down for the winter. This was the first test after setting up the masts, maybe even after the antenna splitter was installed to my friends boat. My friend still hears well distant broadcasts from official stations and his AIS receives ships even up to 70 nm. So I was certain there is something with my sending. I climbed to the mast and took the antenna down. It is a 90 cm Glomex steel antenna and seems to be short circuit when measured with a multimeter (well below 1 ohm). I found some rather poorly done UHF connectors, which I soldered (the outer mesh was only pushed in place) and also one 90 degree piece that had 1 ohm DC resistance for the middle pin. I measured the cabling and it had more than 20 Mohm between the pins without the antenna and 1.3 ohm with the antenna. I also opened the cover of the antenna and it was so simple that it's hard to believe there would be something wrong in it. Just a few rounds of copper wire around a plastic and a connection to the steel whip.

Testing again with my friends boat gave the same result. I did some test to a handheld radio. I could hear 3 nm while the handheld was sending at low power (1 W? It has high/mid/low) and the handheld could only hear while I was sending at 25 W (not at 1 W). So I thought there must be something wrong, since I needed more power.

While my friend went sailing I did some more test. He could not here me even at 3.5 nm. Then I set up my emergency antenna at 5 m above water. At the same time my friend picked up his handheld. He still could not here me with his mast top antenna + fixed VHF, BUT he could here me with his handheld, even when it was just lying on the cockpit seat. So he seems to have a problem with his reception and maybe I have a problem with my sending.

I ordered a new cable, Aircell 5, to replace the 21 m of RG58 and new connectors. I haven't yet installed the new cable.

I also did some VSWR measurements (Sheakspeare) of my system. When I connected the meter at the radio, It showed 18 W power and 1.6:1 VSWR. When I removed the 1 ohm 90 degree connector at the radio I got 20 W and 1.5:1. I also measured at the connector near the base of mast (below floor) and got 18 W and 1.2:1. My emergency antenna showed 22 W and 3:1, so it seems to need some tuning. The emergency antenna is just a dipole made out of RG58.

So is there anything wrong with my system? Can the cabling or the antenna be bad even when I get good VSWR readings? Is it normal to need to use 25 W while the handheld only needs to use low power? Can there be something wrong with the radio?

What about my friends boat? Can the active antenna splitter somehow spoil RX while TX still seems to work very well? Why can he still hear VTS, weather broadcasts etc?
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Old 02-07-2016, 02:34   #2
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Re: VHF mystery

Unlikely, but depending on the channel sometimes you get a similar duplex vs simplex problem if your set changes from US to International.

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Old 02-07-2016, 02:45   #3
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Re: VHF mystery

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Unlikely, but depending on the channel sometimes you get a similar duplex vs simplex problem if your set changes from US to International.
We are in Europe and used channel 77,which is meant for vessel to vessel traffic and is a simplex channel (156.875 MHz).
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Old 02-07-2016, 03:04   #4
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Re: VHF mystery

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We are in Europe and used channel 77,which is meant for vessel to vessel traffic and is a simplex channel (156.875 MHz).
Yeah ok, not that then, good luck finding out whats wrong. Cheers
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Old 02-07-2016, 03:15   #5
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Re: VHF mystery

Just heard that my friends boat seems to have a very recent reciver failure. He hasn't been able to receive anthing during the last 24 hours. Not even while bypassing the splitter or using the emergency antenna. AIS reception is still OK. He said it worked OK before and after our first test about a week ago.

So it is possible that I had a problem with some of the connectors and it is now fixed. But I think I'll change the cable anyway, since I don't like the connectors in the current cable and it has suffered from UV at the top.
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Old 02-07-2016, 05:26   #6
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Re: VHF mystery

I believe in Europe channel 77 is low power 1W only? You need to try a high power simplex channel. Also, you need a pigtail adapter to connect your handheld to the main antenna coax so that you can test better as well as gain more redundancy.


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Old 02-07-2016, 05:28   #7
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Re: VHF mystery

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...I also did some VSWR measurements ...measured at the connector near the base of mast (below floor) and got 18 W and 1.2:1....

Can the cabling or the antenna be bad even when I get good VSWR readings?
Any VSWR measurement made away from the antenna will be affected by loss in the transmission line. Let me explain this in a simple way. We assume a transmission line that is very lossy, an antenna that is completely broken off, and 25-Watt transmitter:

Your transmitter sends 25-Watts into the transmission line. The line has a lot of loss, say -10 dB, so only 2.5-Watts reaches the antenna.

The antenna is not even connected to the transmission line, so the actual VSWR at the antenna is infinite, and all 2.5-Watts is reflected back to toward the transmitter.

The 2.5-Watts of power reflected from the antenna is also attenuated in the transmission line, so only 0.25-Watts reaches the VSWR meter.

The VSWR meter reads the following:

Incident power = 25-Watts
Reflected power = 0.25-Watts

This gives you a VSWR of 1.22:1

You think, hey, swell, everything is good. The real situation is your antenna is not even connected.

In your situation the RG-58 transmission line will be introducing some attenuation. While perhaps not a -10 dB loss, there is some loss. Any loss tends to mask the real VSWR at the antenna.
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Old 02-07-2016, 05:38   #8
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Re: VHF mystery

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Just heard that my [friend's] boat seems to have a very recent [receiver] failure.
This illustrates the problem of testing your radio by using someone else's radio as the test instrument. You assume the other guy's radio is working properly. You assume if there is a problem in the test communication it must be caused by your radio. These can be bad assumptions.

You can often resolve this ambiguity by testing with two other stations. With three stations at mutually equal distances, you can usually resolve which station has a receive or transmit problem.

Also, in any testing of a communication circuit, you really need a back-up and working communication method between all stations so you can easily exchange information on the devices under test.
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Old 02-07-2016, 06:56   #9
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Re: VHF mystery

jmaja,

I had a similar problem with weak VHF. Boats near me could contact a shore station when I could not. A couple of weeks ago I put my SWR meter between the VHF radio output and the coax leading to the mast and measured 26W forward and 0.05W reverse on ch 66 high power. Then I moved the meter to between the coax and the antenna at the top of the mast. There I measured 5.7W forward and 1.2W reverse on ch 66 high power. I plan to replace the cable and antenna this fall.
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Old 02-07-2016, 09:58   #10
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Re: VHF mystery

I replaced the cable today. Now I get 1.2-1.3:1 VSWR, 20 W and 0.74 ohm DC resistance at the radio end. According to the cable datasheet 0.74 ohm is excactly the sum of the core and shear resistances of my 22 m cable. So everything is a bit better. I haven't been able to do any other test than the same 3 nm test to a handheld which was as before.

But now I also found out that the handheld had a software squelch setting (not mine, thought it had an automatic squelch) and it was at maximum. Probably that caused 1 W not being heard. Will try again tomorrow.
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Old 02-07-2016, 10:44   #11
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Re: VHF mystery

jmaja,
Good that you replaced the coaxial cable (and all connectors as well, I assume?) and do not use the 90* elbow connector, as most of them are very poor!!
And while your use of Aircell 5 is okay, it is similar to Times LMR-195, understand that your 22m length will have about 2.6db of loss...acceptable, but not outstanding....

In addition to the excellent testing that you have done, on a pleasure boat in Europe, your best bet for "on-air testing" is with known good stations, at repeatable distances....other boats are okay, but as others have mentioned using another boat whose unknown radio system is also suspect can lead you going-in-circles looking for a problem on your boat that might not exist!

Having an emergency antenna is a good idea and using this as part of your testing is very good.....another good way to eliminate your radio from the possibility of being at fault is to do as Jedi suggested, have a small coaxial jumper cable that allows you to use your handheld with your masthead antenna...


In any case, I think you will enjoy reading these two threads...
VHF and AIS Radiowave Propagation and VHF and AIS Radio Range

Vesper AIS SP-160 "relay/splitter" test results, lab/real world



I hope this helps..

John
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Old 02-07-2016, 11:55   #12
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Re: VHF mystery

Yes I replaced all the connectors. Now I have a BNC pair near the base of the mast (well away from possible water in the bilge) to allow removing the mast for the winter and a solder/crimp 90-degree UHF connector http://paratronic.fi/kuvat/e2703.jpg at the radio. All the connectors are gold plated unlike the old ones, which already had some corrosion.

There is so little room behind the radio, that a 90 degree is a must, but now I just have the connector, not a straight connector + 90 deg adapter. At the mast there is a special connection to the Glomex antenna, which uses no actual connector.

I think it is good to test with other boats that are far enough. If everything works, everything is fine. Testing with a coastal radiostation can give a too good results due to extreme antenna height and gain thus letting you think everything is OK while it is not.

Yes, I know I'm still losing quite a lot of power at the cable, but much less than I used to with RG58 (4 db just for the cable) and more lower quality connectors. I didn't want to use a thicker, heavier and way more expencive cable. Aircell 7 would have been slightly better, but a real difference would have needed a high quality 10 mm cable.
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Old 02-07-2016, 13:18   #13
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Re: VHF mystery

vhf in Europe? 6 years in the Med and hardly hear any chatter on 72 or 16 let alone when scanning all channels. Only ever use it 1/4 mile from a marina that publishes a contact channel. Most contacts while cruising in the Med by email and mobile phone. BTW we often Skype the family back in Oz when we're up to 5 miles off shore.
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Old 02-07-2016, 23:22   #14
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Re: VHF mystery

I sail in the Baltic Sea. I use VHF very seldom. It's a safety feature. Mobile phone can be used only up to 10 nm from coast and doesn't help contacting the boats near by. Not much happening on 16, mostly reminders of safety and weather reports on other channels, but quite a lot of traffic on VTS channels.
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Old 03-07-2016, 14:54   #15
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Re: VHF mystery

I just tested to send from the boat to the handheld the same 3 nm distance with different squelch levels. Thus Icom IC-M71 has a 11 level squelch. 0 is of, already at 1 there is no noise and 10 is maximum. 8 was the highest level that worked while I was sending at 1 W power. So there seems to be quite a lot of marging. Any idea what that level might be in dbm? For a free space the path loss would be 91.5 db, but it's not a free space. There are at leas plenty of trees maybe even the small hill in between blocks the line of sight.

Need to test a much longer distance at sea, but it is not that easy to find someone to test with.
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