VHF issue

[SCORPIO]

I have two fixed mount radios on my boat, both only a few years old. One is an Icon IC M412 the other a Standard Horizon Eclipse. I was noticing the SH unit not being very active on scan so I got a Shakespeare ART 3 meter to check the antennas. Both are 8 foot Shakespeare units one is an 8300 the other I don't know, no markings. The SH unit is connected to the 8300. After testing, the SH antenna has an VSWR way over 3, confirmed by swapping to the ICOM unit. I replaced that antenna today with a Shakespeare 5225 and I still have a high VSWR reading on it. All connections are new Shakespeare crimp style connectors, same as on the other good antenna. If I understand the meaning of the VSWR number, anything over 3 and I'm looking about 50 percent of my transmit power. I don't know what to look at next. Anybody have any suggestions?

 

[Bt Doctur]

Has anyone cut or spliced the coax?

 

[ttmott]

The gain on the antennas is related to receiving and transmitting distances. The gain you choose is related to the type of boat and where you boat. A gain of 3db, for example will provide a wide (vertical) capability but range will be shorter. On the other hand a gain of 9db will have a very narrow beam but range will be longer. You can compare to a flood light (3db) and spot light (9db). So check that both antenna are same gain. I use 6db which work great for the type of boating I do.

 

[SCORPIO]

Both are 6dB. There is a splice in the cable, I replaced it twice today, no difference.

 

[be_prepared]

Is the cable that came with the new 5225 too short to reach the transceiver without a splice? The spliced cable could be contributing to higher SWR. I know sometimes the splices are because it's hard to fish the new wire all the way through the boat, so we compromise, but a contiguous run is a better way to get the most signal to the antenna if possible. Every connection causes loss.
FWIW, Most marine and amateur transmitters have protection circuits that will reduce power output at high VSWR.

 

[Bt Doctur]

You can never"splice" coax. It` needs the PL259 on both ends and a SO239 in the middle. So the best connection is a soldered connection versus the crimped type.
Any type of ant can be tuned for the lowest SWR reading using a swr bridge and cutting the coax a 1/2 inch at a time Should the swr`s increase keep cutting until they start decreasing, Should you run short, connect a section in multiples of about 6 feet. A radio dosent know how far the ant is an long as its a multiple of it`s wavelength .VHF is 6 Feet so any multiple of 6 feet should give a 1:1 swr.

With a little work your cheapest radio putting out 25W will sound like a linear

 

[SCORPIO]

Thanks for the info BTDOCTOR. The connection is with one of the Shakespeare crimp type connectors. I'm thinking that may be the issue. I may need to find a new way to route the cable to avoid any splices.

 

[Phasma]

Always avoid splicing coax. The SWR measures reflectivity. An antenna is tuned to the length of the wave transmitted through it. If the antenna is to long or two short then a portion of your transmitted signal is reflected back into your radio. Really tough on your finals. Now a days unless the antenna is physically damaged it is rarely an issue as antenna come pre-tuned from manufacturer. Coax should be one continuous length as short as possible. Don’t coil any excess. Trim it and then ideally solder a PL259 onto it.

Side note: your antenna should be perpendicular to the water. I see so many boaters with their antennas at strange angles for appearance purposes. Your antenna was designed to work straight up and down.

 

[Bt Doctur]

it is rarely an issue as antenna come pre-tuned from manufacturer. Coax should be one continuous length as short as possible. Don’t coil any excess. Trim it and then ideally solder a PL259 onto it.

I must disagree, the ant comes with 20 feet or so and is recommended to not shorten it. That is their recommendation to have it work with anyone`s radio .The absolute minimum for length is 6 feet from the radio plug to the ant
Coax is measured for the wavelength To get a lowest reading the length must match the radio to ant length. Coiling is not an issue provided you have a large enough of a coil .A minimum of a 10 inch dia is perfectly acceptable

If my ant is 6 feet or 600 feet in multiples of 6, the swr will remain the same