AM/FM Antenna - Puzzled by Impedance

[Namoian]

There is that old saying that a little knowledge is a dangerous thing. Our boat currently has a nice Alpine (AM/FM/Satellite/ipod/video) radio that being mounted inside an aluminum boat, gets poor reception.

To cure this I decided to install an external antenna and spoke to Alpine tech support in LA who advised that the antenna impedance spec for the radio is 75 ohms - it seems most am/fm radios are 75 ohms. I bought some RG-59 cable which is rated at 75 ohms and a nice 5' Glomex am/fm antenna to mount at the stern. The puzzling part is that while most radios call for 75 ohm antennas, most whip antennas are rated at 93 ohms??? To confuse matters more, many people connect their am/fm to their VHF radio antenna using RG-8 (and a splitter) which is rated at 50 ohms.

I vaguely recall from my HAM course that impedance miss match is bad although not so critical when receiving. Someone out there probably knows the real story.

[Wotname]

Originally Posted by Namoian

There is that old saying that a little knowledge is a dangerous thing. Our boat currently has a nice Alpine (AM/FM/Satellite/ipod/video) radio that being mounted inside an aluminum boat, gets poor reception.

To cure this I decided to install an external antenna and spoke to Alpine tech support in LA who advised that the antenna impedance spec for the radio is 75 ohms - it seems most am/fm radios are 75 ohms. I bought some RG-59 cable which is rated at 75 ohms and a nice 5' Glomex am/fm antenna to mount at the stern. The puzzling part is that while most radios call for 75 ohm antennas, most whip antennas are rated at 93 ohms??? To confuse matters more, many people connect their am/fm to their VHF radio antenna using RG-8 (and a splitter) which is rated at 50 ohms.

I vaguely recall from my HAM course that impedance miss match is bad although not so critical when receiving. Someone out there probably knows the real story.

Just a quick simple answer, others may want to add detail .

Comms systems (VHF, SSB, UHF, FRS etc) always (OK, almost always) use a 50 ohm characteristic impedance.

TV and some satellite (and I believe FM radio) use 75 ohms.

Always match the cable, connectors and antennas to the desired system.

The HAM crowd make all sorts of antennas and feed lines and matching networks but I will let them explain their stuff

[SoonerSailor]

For receiving, this impedance mismatch is inconsequential. Even for transmitting, a 75 ohm to 93 ohm mismatch is of little consequence.

[Wotname]

Originally Posted by SoonerSailor

For receiving, this impedance mismatch is inconsequential.

While somewhat true, you will get better broadcast FM reception using a matched antenna and feeder; AM, maybe not so much.

Originally Posted by SoonerSailor

Even for transmitting, a 75 ohm to 93 ohm mismatch is of little consequence.

What transmitters are you using that have a 75 ohm impedance and what antennas are 93 ohms? Not doubting you, just curious.

[SoonerSailor]

A 93 ohm to 75 ohm mismatch would result in an SWR of 1.24:1 and a 99% match efficiency. You could not detect a difference between this and a perfectly matched antenna at LF, MW, HF, VHF, UHF and beyond without expensive test equipment, and certainly not with any normal receiver.

Even with a transmitter, where SWR and impedance matches are far more consequential, this would be considered a very good match, not a significant mismatch. Our radio transmitters typically have 50 ohm output impedances, but this is no magic number. A cable TV amplifier would be a transmitter with a 75 ohm output impedance, FWIW.

On top of all that, although they say their antenna has a 93 ohm impedance, that would only be at one frequency. Higher and lower frequencies will show different impedances. It might well be 75 ohms at 88 mHz and 110 ohms at 108 mHz but that still would not result in significant performance differences in reception across the band.

Mismatches will have to be much higher than this to affect receiver sensitivity to any perceptible degree even on the relatively quiet bands above 30 mHz.

Chip

[Namoian]

Thank you for this – I will go pull that 75 ohm cable through the boat and hopefully cross another item off the list.

When the boat was built, the fm receiver was attached to the VHF antenna on the mast top using a Shakespeare splitter. It didn’t work very well, perhaps because the antenna cable run was long at 90 feet, perhaps because of the impedance mismatch between 75 ohm fm radio and 50 ohm VHF system and perhaps some signal decibels lost in the splitter itself. I get excellent VHF radio performance but had it tested with an SWR meter just in case. I also pulled the fm radio and had it tested – it was OK. Next I tried an internal automotive powered antenna which gave better, but still not great performance in a metal boat (possibly good solution for fiberglass).

By the way, the fm radio tech suggested using a signal booster if the antenna cable run is greater than 30’ which I should not need going to a whip antenna on the stern.

Cheers,
Ian

[Wotname]

Ian, it sounds like you are on the right track, it should work out well.

Just to add to the point your FM tech said about adding a signal booster; an alternative solution would be to use a higher grade coax i.e. a lower loss coax.

The RG59 while fine for short runs (say >20 ft), something like RG11 is more suitable for longer runs.

The attenuation of RG59 is about 10dB per 100 ft while RG11 is about 3.5dB per 100 ft; note, both have a characteristic impedance of 75 ohms.

Good luck with the project!

[hellosailor]

Ian-
A powered antenna (active booster) no matter how cleverly engineered, is always going to boost the noise along with the signal. So, better to use low-loss cable and a good antenna. Car stereos aren't very fussy about antennas, although especially for the AM "more is better" and of course that conflicts with having a less obtrusive antenna. I suspect that getting anything on the outside of the boat will make a world of difference.

[Lake-Effect]

Idea - before drilling holes and running any wire, just try the antenna and a short length of cable and confirm that it all works.

[aressiont27]

Originally Posted by Wotname

Just a quick simple answer, others may want to add detail .

Comms systems (VHF, SSB, UHF, FRS etc) always (OK, almost always) use a 50 ohm characteristic impedance.

TV and some satellite (and I believe FM radio) use 75 ohms.

Always match the cable, connectors and antennas to the desired system.

The HAM crowd make all sorts of antennas and feed lines and matching networks but I will let them explain their stuff

There should not be any difference while receiving, but for transmitting 75 to 93, a bit of only. It might be sorted just through a realignment of the connection. May be, The sea is causing a problem here instead.

[GordMay]

Greetings and welcome aboard the CF, aressiont27.