VHF / FM seperator

[walt]

I did a search on this forum and didnt find exactly what I was looking for so this post.

Im looking at this VHF / FM seperator (on this site) http://shop.macgregorowners.com/detail.htm?fno=80&group=1059

I have a single VHF radio, single VHF antenna and an AM/FM radio. The seperator allows both the VHF radio and the AM/FM radio to use the antenna - at the same time. No mechanical switching is required. All the discussion I found was on some device which I beleive needed to be mechanically switched - which Im not interested in.

What this device (diplexer or seperator) should do is provide proper impedance matching of both radios to the antenna with low loss but prevent the output power of the VHF radio from being applied directly to the FM radio input (which would likely not be a good thing).

This is a passive device (ie, no powered electronics) so should be good and reliable. However, it it were not reliable, it would add risk of damaging the radios.

Does anyone who uses one of these have any feedback? How well did the FM radio work since the typicall VHF antenna is not optimized for FM radio?

 

[Brian D]

I have that device. I put it in line and it did nothing for my FM reception. Currently my FM radio is connected to the shrouds. The reception of a local Jazz station I get 50% of the time did not improve. So this device did not work for me. Now, I may have a faulty antenna or possibly bad coax. So it could be an installation issue and not a device issue.

I think the best option is to use a real FM antenna. You will get the best match for the frequency band 88MHz to 108MHz and not a tuned circuit to make the 170MHz band work at lower frequencies.

 

[Dave Groshong]

It is not intended to enhance your reception, just allows one antennae usage for both VHF and AM/FM. Some adjustment of your radio may be necessary for the AM side, but FM is unaffected, unless your run is longer than the 10ft cable provided.

Here's a link to the Shakespeare page:

http://www.shakespeare-marine.com/antennas.asp?antenna=4357-s

As with any other product you purchase here, your satisfaction is guaranteed.

 

[sailingdog]

I would recommend getting a separate AM/FM radio antenna instead. It doesn't usually have to be mounted very high, although higher up gets you better reception in some cases.

The problem with that splitter is that it is one more thing that can fail. I prefer to keep my emergency communications equipment separate from non-emergency equipment and as simple as possible to reduce points of failure. Also, if it fails in a certain way, it can allow the VHF broadcast output to enter the antenna of the AM/FM radio, which will more than likely fry the radio's circuitry—as it isn't designed to deal with 25 Watts of radio energy.

 

[walt]

These devices are passive - just some inductors, caps, resistors, traces, ect. A lightning strike would probably damage the seperator (which might protect the radio - who knows) but this type of device should be very reliable. They should be very robust to ESD, ect and the VHF transmit power should have no effect. Id worry about connectors failing - and you have introduced more connectors into the system adding the seperator.

I have been using an FM antenna already but busted the connector. Just exploring options..

 

[Neil McDonald]

I would recommend getting a separate AM/FM radio antenna instead. It doesn't usually have to be mounted very high, although higher up gets you better reception in some cases.
.

So what defines a proper AM/FM antenna? I inherited an auto style antenna lying flat on a shelf just behind the AM/Fm receiver!

 

[sailingdog]

I use one that looks like this one from West Marine.

 

[Neil McDonald]

Mounted?

I use one that looks like this one from West Marine.

Mounted above deck somewhere I assume? My AM/FM is down below at the nav station.

 

[sailingdog]

Actually, it is mounted right at the nav console... works fine, even for stations I can't get on the radio in my truck in the marina parking lot.

 

[Scott On Eagles Wings]

I have the Shakespear Splitter

I have been very happy with it, get good reception with both radios. Easy to install, less wires running around the boat and have had no problems whatsoever after four seasons.

 

[caguy]

I use an antenna that came with the radio it is about 3"x18" and embedded in thin plastic that sticks to the window I have it stuck to the window in the head behind a curtain. The window is tinted so you don't see it from the outside either. It works well, I can get FM all across the Catalina Channel.
Frank

 

[Neil McDonald]

So what defines a proper AM/FM antenna? I inherited an auto style antenna lying flat on a shelf just behind the AM/Fm receiver!

Tonight I talked to a retired communications engineer about how poorly my present AM/FM antenna setup works. He immediately asked if I have problems with both AM and FM, to which I replied only AM. He said that my problem is likely that a horizontal aerial will effectively pick up FM signals but normally not AM. He also said that some communication systems have even been designed to simultaneously transmit two quite different signals from the same station using the same frequency - with one needing a vertical aerial and the other a horizontal aerial at the receiving station.

So before I do anything else, I am going to experiment with the same aerial set vertically.

 

[Bob Poff]

AM Radio Stuff

Your friend is correct. Most FM transmitting antennas (and TV for that matter) radiate with horizontal and vertical patterns. Or as the stations I used to engineer did, circular polarization.

AM meanwhile is radiated vertically polarized. The towers you see at an AM site ARE the antenna. But with reflections,things re-radiating and such, I don't think polarization is really that much of a problem.

Some things that most likely are getting you:
Antenna size.
A full size (1/4 wavelength) antenna at 910 KC (AM) is almost 260 feet long.
Meanwhile the equivalent antenna at 103.3 MC (FM) is a little over 2 feet long. So your marine antenna through one of those couplers has an advantage being closer to the correct length. (I never did figure out how they get car antennas to work so well on AM).

The AM band is noisy.
Much much more than years ago. Between the crowded channels and just a general increase in background noise (power lines, computers, street lights and so on) the AM radio is at an immediate disadvantage. Add to that the fact that noise is generally an AM (amplitude) type signal. FM by looking at the frequency changing to retrieve the audio is more immune to that noise.

Last of all...... On most newer radios, the AM receiver is LOUSY! It's like the manufactures grudgingly say if we gotta have AM.. put in the cheapest bloody thing possible.