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Radio antenna orientation

  1. Sep 14, 2014 #1

    anorlunda

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    The AM/FM radio on my boat has a simple wire about 70 cm long as an antenna. It is much less effective than a car antenna. I say that because it has difficulty picking up stations that the car gets easily.

    At sunrise/sunset the effects of radio propagation become evident as the stations fade out entirely and no jiggling the antenna will help.

    Most puzzling, when listening to a weak station, I orient the antenna to get a clear signal. Then at irregular intervals varying from a minute to several hours, the sound fades out suddenly and I have to reposition the antenna to get it back. I move it left, right, up, down, bent straight ... a random orientation in 3D. It's does not seem to matter whether the boat is swinging at anchor, or tied in a fixed position to a dock.

    My first question, what is going on with these sudden fade outs, and why does a random reorientation of the wire help?

    Second question, how can improve the reception performance of this radio and antenna to match that of a car? Even the old fashioned whip antennas on old cars with 70cm length and fixed orientation seem to outperform this.
     
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  3. Sep 14, 2014 #2

    davenn

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    hi there

    The varying way the ionosphere reflects the radio signal causes changes in polarisation to that signal
    That and the changing density of the ionosphere is what causes the signal to fade and return


    if you have a yacht and a metal mast, use it for the antenna, else just run out a longer length antenna wire


    cheers
    Dave
     
  4. Sep 14, 2014 #3

    anorlunda

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    That makes sense, reorienting the wire matches the polarization.

    How then do vertical fixed car antennas avoid the problem?
     
  5. Sep 14, 2014 #4

    sophiecentaur

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    The ionosphere has very little effect on terrestrial propagation of radio waves at VHF frequencies - they mostly just pass straight through and go into space. Occasionally, you can get 'Sporadic E' propagation (the clue is in the name lol), which can cause VHF signals to propagate very long distances but it is not a reliable way to serve a radio audience. It doesn't effect the main service area of the transmitter, afaik, but just causes remote interference. The E layer is at a low altitude and has a high density of ions, which can reflect VHF signals at an oblique angle and bounce them way over the horizon. I think the daytime presence of the D layer usually causes the signals to be absorbed. It's a complicated and very statistical business. No wonder they avoid Ionospheric propagation whenever they can - and use satellites.

    There are differences between a car antenna and a boat antenna. Both antennae will be Vertically Polarised (to obtain an omnidirectional pattern) but they may be mounted at different heights (boat - marine radio comms - antennae are often mounted high on a mast or a high point on a cabin top but only communicate at low power and only make 'line of sight).

    I have noticed a similar effect on my boat (down in the cabin), using a portable VHF broadcast receiver well within the service area of the local station. This is with the set more or less on the level of the water. I expect that most of the received signal comes from re-radiation of the steel rigging plus mast, which will behave like a large random spaced array. This will have a crazy sensitivity pattern (HRP) and will likely produce some deep nulls in some directions. I get deep fades just when I am trying to listen to something important. I have not bothered to fit a proper antenna, high up, which would probably solve my problem.
    Car antennae also have 'fried egg' patterns but not as bad, as the antenna tends to be mounted at the highest point and over an earth plane (roof).

    The path to your boat antenna will probably be over sea, which produces a strong reflection. This reflection can interfere with the direct signal and cause fading, as a boat moves up and down on waves and, of course, as the tide rises and falls, the difference between the direct path and the reflected path will change* (the old Young's slits thing again but in a different guise) and you can get additive and subtractive addition of the two signals when their amplitudes happen to be similar. Tidal fading is particularly an issue for UHF propagation but I believe it does happen at VHF too, but the wavelength is longer so the interference pattern is broader.

    Cars often have a problem with multi path propagation due to nearby hills and buildings but the transmitters are often sited more to suit cars than boats.

    * I know the boat goes up and down with the tide but the transmitting antenna doesn't!
     
  6. Sep 14, 2014 #5

    berkeman

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    Time to invest in satellite radio entertainment, eh? :smile:
     
  7. Sep 15, 2014 #6

    jim hardy

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    70 cm is almost a quarter wavelength in US FM band 88 to 108.
    You might try tripling the length of your antenna.

    Used to be that automobile radios had very good "front end" circuits for the reasons mentioned above. I once hooked the AM tuner from a '51 Chrysler to a stereo. Picked up Montreal in the daytime, from central Missouri.
    So i took a look at whats up these days. Sure enough, automotive FM receivers have gone high tech.
    http://www.silabs.com/Support Docum...ith-CMOS-Based-AM-FM-Automotive-Receivers.pdf

    If you shop for a radio look at the sensitivity.
    It's given as microvolts or db. Lower is better.
    http://www.silabs.com/Marcom%20Documents/WhitePapers/Automotive-Radio- [Broken]
    Key-Requirements.pdf
     
    Last edited by a moderator: May 6, 2017
  8. Sep 15, 2014 #7

    sophiecentaur

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    It's difficult to make firm statements about general EM propagation and reception because all the different frequency bands behave pretty differently.
    Trouble with using a longer antenna (3λ/4) is that any match you may (pretty randomly) achieve, could be more frequency dependent. The low tech way to improve antenna performance is first to get it as high as possible. 'Height gain' is always good value. I don't know how much your boat will rock and roll but a vertically directive antenna can have a too narrow beam and it can point up or down, away from the wanted signal. VHF wavelengths are too short to make masts and steel rigging work well as antenna because you just don't know what's happening to the currents flowing around them - hence the pattern can be really poor / irregular (It could have a null right in the horizontal direction). A tall mast, as a receiving antenna, is good for MF, of course and a lot better than a car antenna, which just samples the field (volts per metre) with well under a metre of 'high impedance probe' wire. 10m of mast could give you around 20dB better pickup.
    There are advantages operating at sea. Fewer local sources of interference and not much (buildings and hills) to get in the way.

    VHF transmissions are mixed polarisation (deliberately, to get the best reception for many antennae and receiving sites) so any changing of polarisation due to multipath propagation is less likely to do much harm.

    But, in the end, it's 'suck it and see', when you don't have RF measurement equipment available.
     
  9. Sep 15, 2014 #8

    NascentOxygen

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    I interpreted OP's description of his signal propagation problems as indicative of the AM broadcast bands. The fading description (sunrise) is not characteristic of VHF FM receivers. Perhaps anorlunda could confirm the band he is finding his distant stations on.
     
  10. Sep 15, 2014 #9

    anorlunda

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    I am the OP. Thanks all for your helpful suggestions. My difficulties are primarily FM reception, not AM, not VHF. (AM reception is always poor, but I'm not interested in that.)

    I have a VHF antenna at the mast head. I get 25 miles line of sight reception always, 300 miles exceptionally. I use the backstay, about 45 feet long for the SSB radio antenna. I talk across the Atlantic on that. My problems are with FM radio. I have a ground plane under all three radios.

    When only 5-10 miles offshore I sometimes get better cell phone signals than FM radio. That's is ironic because I can listen to local radio over the Internet, but not directly via the FM radio.

    When I have problems at sunset it is usually when the sun is nearly on a line between me and the station.

    The radio sits only 2 feet above the sea!!! Maybe that is why a cheap AM/FM clock radio with no external antenna in a house outperforms my radio on the boat. If I had a trawler rather than a sail boat, the radio would be mounted 6-10 feet above the water. There is little I can do to mount it higher.

    I've wondered if the solution should be to buy a auto radio as a replacement, even though they are not corrosion resistant in the salt air. But if the problem is antenna, that may not help.

    Perhaps I could run an experiment and temporarily hook the masthead antenna to the AM/FM radio instead of the VHF.
     
  11. Sep 15, 2014 #10

    NascentOxygen

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    Your FM stations broadcast outside the VHF bands? What frequency are these FM stations transmitting on?

    Broadcast stations set their signal pattern to match their anticipated audience. Quite likely the station antenna directs most of its power over nearby towns and villages, and deliberately radiates little out over open water because there would be few listeners there.
     
    Last edited: Sep 15, 2014
  12. Sep 15, 2014 #11

    anorlunda

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    For example 90.3 MHZ FM. The commercial FM band.
     
  13. Sep 15, 2014 #12

    NascentOxygen

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    That's VHF.
     
  14. Sep 15, 2014 #13

    anorlunda

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    True, Wikipedia says "Throughout the world, the FM broadcast band falls within the VHF part of the radio spectrum. Usually 87.5 to 108.0 MHz is used,[1] or some portion thereof, with few exceptions:"

    But marine VHF uses 156-162 MHZ.
     
  15. Sep 15, 2014 #14

    anorlunda

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    Last edited by a moderator: May 6, 2017
  16. Sep 15, 2014 #15

    sophiecentaur

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    @anorlunda
    It would be interesting to see if re-siting your antenna at the mast head has any good effect; the match of the comms antenna wouldn't adversely affect reception much at 100MHz. I suspect it could make all the difference to your reception problems.
    If you wanted to have two antennae up there, you might be best to site the broadcast antenna below the Marine Radio antenna. 20W from the other antenna could give the front end some problems if they were at the same height but, offsetting them vertically would remove that problem. I'd bet your broadcast antenna could work fine if placed at spreader height (on a spreader?) without causing any serious null - particularly if you tilted it a bit to receive mixed polarisation. Many people site a receive antenna (or comms spare) on the pushpit, which is significantly higher than the saloon table.
    A car radio would be much better than what you are using, I'm sure. Look on eBay for something affordable and keep it covered in a plastic bag if you are worried about the dreaded corrosion. I must say, my portable has lasted quite a few years so far.
    Do keep us informed about how it goes.
     
  17. Sep 15, 2014 #16

    davenn

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    yes exactly which is why I responded the way I did
    Pity we didn't have the clarification earlier :wink:

    Dave
     
  18. Sep 15, 2014 #17

    sophiecentaur

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    Marine traffic is not 'Broadcast"; it is Communications. Broadcast is essentially One Way transmission. Radio Band 1 and Band 3 are also referred to as VHF and were used for analogue TV Broadcasting in the old days. Nowadays the bands are bid for by the big money users.
    The classifications are confusing for the uninitiated and the broadcasters do not help in their announcements.
    There is LF, MF, HF, VHF, UHF, SHF (and more?) and they are all specified but with fuzzy edges, depending on who you are. I think the ITU is the ultimate authority on these classifications.
    It's amazing how frequencies from 3 to 30 MHz were regarded as 'high' in the old days. GHz operation was unheard of until quite recently - but now we have tiny hand held devices that use processors operating at several GHz.
     
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