This antenna on my roof - why bad reception?

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gewecke

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I bought this cheap antenna Amazon.com: TRAM 1600-HC 38- Inch VHF Marine Antenna: Electronics and stuck it on my roof to monitor the marine band. Reception is a bit less than I had hoped, considering I have a decent line of sight with most of the bay I wish to monitor. I was thinking - does this antenna require some sort of ground plane to receive better?
What kind of cable do you have on it? That makes all the difference. :wink:

73,
n9zas
 

SCPD

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As a general rule marine antennas are designed to work without a ground plane. I would consider going to a better grade of feedline.
 

kb2vxa

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I can't see what sort of connector that thing has so I have no idea what sort of coax you can use. I was thinking along the lines of LMR400 but there are unanswered questions.

Eh, the antenna leaves much to be desired and Jimmy's general rule doesn't apply to 1/4 wave ground planes. Mounted on some metal structure of a boat it would have a crude planing surface but mast mounted as a base antenna it's only half an antenna system. You may drill holes in the corners of the horizontal part of the mounting angle bracket and mount 4 38" lengths of 1/16" diameter brazing rod as radials, that will surely improve things but don't expect miracles. Stainless steel bolts, nuts and split ring lock washers will do the trick nicely, don't use cad plated crap hardware outdoors that will turn to rust in short order. Don't use copper wire for the radials either, it's soft and will turn into a pretzel in the wind. Brass is stiff and short lengths will withstand a hurricane.

You have another option, a good and proper marine antenna that doesn't require a ground. They have a bit of dB gain to them while a ground plane has unity gain, nil. In any case consider the coax, on a boat it's a short run so a bit of signal loss is tolerable. For a longer run such as you have loss becomes a consideration, essentially you need better coax than what came with the antenna.
 

mmckenna

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Those are often a 1/2 wave antenna design and meant for use on the very top of a sail boat mast. Generally you don't want a real high gain antenna on top of a sail boat mast. A high gain antenna with a tight pattern aimed at the horizon can be detrimental when the boat is taking rolls. Traditional thinking is that a lower gain antenna on a sail boat mast with a more of a even pattern will work better. Enough with the theory....

1/2 wave antennas will exhibit unity gain (0dB) without a ground plane, and about 2.5dB with a ground plane. You can choose to run it either way.

If you have a good view of the bay, then it should be working well, the amount of gain should't matter so much. I'm likely to agree with the others, might be a coaxial issue. Let us know what you have.
 

majoco

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Make yourself a vertical dipole out of an old low band TV antenna - make the elements 20" long each side and they should match into 75ohm cable no worries. No ground plane required - reception pattern will be like a half-donut cut horizontally - the centre of the hole is you!
 

kc9jmd

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I'm using RG-8x "marine grade" cable because that's what I had lying around. It's about a 50' cable run. I'm open to using a different antenna, this is just a crude setup based on gear I already had. Again, this station is for receiving only, as it is shore- based. ;) any ideas on a different true base antenna?
 

nanZor

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Hard to find real specs on that to see if it is just an end-fed 1/2 wave, or a 5/8 wave (electrically, rather than just a length measurement) Frustrating.

If it is just an end-fed half wave, not much of a ground plane is needed. If it is actually a 5/8 wave, then a good ground plane is needed to keep the lobes low. I'd make a coaxial choke or a bunch of #43 mix ferrites just below the feedpoint too.

What are you using to receive this with? It is possible that you are now suffering overload / desense if your current setup can't handle out-of-band signals, like pagers, fm-broadcasters, NOAA weather stations etc. Got any fire-breathers in these areas now that just laugh at any sort of attenuation?
 
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SCPD

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If you zoom in on the picture it actually says 1/2 wave. And it sure looks like a PL-259 connected on the bottom.

I couldn't find any manufacturer info.
 

Halfpint

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Personally, I'd dig through a closet or two and find 4 heavy wire coat hangers, clip off the hooks and twisted ends, straighten them, and make a loop on each one to fit the appropriate sized screw(s). (Not worrying about the length right now as this is really only a test.)

Next I'd bring the mount inside and drill appropriate sized holes on the corners of the mount matching whatever sized screw had been chosen and run a screw through each hole one time to check the fit and to somewhat clean up the holes. Then I'd loosely attach the `groundplane' hanger wires and go back and reassemble the antenna making certain that the `groundplane' wires are making good contact with the bracket.

Finally, I'd give the whole `lashup' *at least* a week or so before I would consider whether there was any improvement before I started trying to make any other `tweaks' to the setup or going `out' and trying to find another antenna.

`Course, this is all supposing that the coax and associated connectors are all in good condition/assembled properly. (I've run across what appeared to be `good' `old' coax with properly assembled connections, and one case without connections, that turned out to have the inner insulation rotting away here and there and it seemed OK when it was hand checked before installation. In the case where we had to attach the connectors it was still on the original spool and we had checked the ends we attached the connectors! [Guess who got `tapped' to climb the tower on that job. {GRIMACE!}])
 

kc9jmd

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