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Old 24-12-2012, 08:00   #1
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SSB Reciever Antenna Length

We have a small Kaito reciever we use mostly for getting weather fax. I have a length of wire run up inside the mizzen exiting at the top where a bare section is clamped to my insulated triatic stay. We get okay reception at night but not very good during the day. I would like to be able to listen to Chris Parker in the mornings but have not been able to. I believe he's on 8.140 at 0700? At any rate I was told that there is an ideal antenna length for 8MHz frequencies and was thinking of making another wire antenna of the correct length for this frequency. Is my logic right here? If so what are the lengths for:

4MHz
8MHz
12MHz
17MHz

Thanks
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Old 24-12-2012, 08:09   #2
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Re: SSB Reciever Antenna Length

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Amateur Vertical Antenna Calculator
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Old 24-12-2012, 10:07   #3
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Re: SSB Reciever Antenna Length

I'm afraid that a tuned antenna won't make much of a difference for your receiver. It won't hurt, but I doubt if you will heart a significant difference compared to a long "random-length" wire.

Here's a test: With no external antenna connected, the whip antenna collapsed, listen to the static "hiss" from the receiver. Then connect your external antenna. If you hear a noticeable increase in noise, then your antenna is doing a decent job. Try this on the different bands you intend to use.

A large part of successful reception is knowing about how frequency, time of day, time of year / sunspot cycle, and location affect radio propagation. Sometimes it's just not going to work, regardless of the antenna.

And if your own boat electronics are generating interference that's going to make reception tough. Make sure you have these basics covered before you spend a lot of energy on new antennas.

You can get good advice on all of these topics right here, so keep experimenting, and let us know how it's going.
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Old 25-12-2012, 08:20   #4
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Sort of what I was afraid of. Thinking the only way I'm going get much better reception is to pick up a proper SSB rig. Right now I can usually get the NOAA faxes from New Orleans and decode with jvcomm32. Outside of that I don't get much. Part of the problem is I don't know where to look. What frequencies should I be searching for things like the BBC broadcasts or Chris Parker or other nets?

Lastly on the antenna length calculator one of the variaes is wavelength. If I'm using a piece of wire and my Kaito radio what wavelength should I enter in the calculator?
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Old 25-12-2012, 08:50   #5
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Re: SSB Reciever Antenna Length

The wavelength of the signal you wish to receive. If you only know the frequency of the signal, then you will have to calculate the wavelength by the usual formula - wavelength (in metres) times frequency (in hertz) equals speed of light (300,000,000 m/s)
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Old 25-12-2012, 08:59   #6
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Ok so for example frequency is 8.140MHz so thats 8,140,000Hz? So wavelength is 36.85? Or do I have my decimals screwed up? Also the site referenced give the following options for wavelength:

1/8
1/4
3/8
1/2
5/8
3/4
7/8
Full

So if I'm trying to calculate the optimal antenna length for tuning to 8.140MHz what parameters should I enter in the calculator?

Thanks.
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Old 25-12-2012, 11:07   #7
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Re: SSB Reciever Antenna Length

If you are receiving static and other environmental noise on the desired channel then your signal strength is sufficient. A better tuned or longer antenna will increase both the noise and signal at the same rate. Receivers intended for HF listening will all be plenty sensitive as that is trivial to design and build.

The only things that will help are things the favor the signal over the noise. I can think of two that relate to boats:

* Move away from shore, away from marinas, boats, nearby high mountains and cities. A mile...more the better.

* Use some sort of directional antenna. Very difficult on a boat. There is a small chance that simply turning the boat to a different heading might help a little.
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Old 25-12-2012, 11:48   #8
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Re: SSB Reciever Antenna Length

Years ago had an signal amplifier that came with a Kenwood reciever I bought at a garage sail. It worked well at boosting the signal but was a bit of a pain to use. The capture bandwidth was quite narrow so useless for scanning the dial. Worked fine if I picked up a weak signal or had a specific frequency that I wanted to listen to. Would tune the radio to a frequency then fine tune the signal with the amplifier. As I recall, it was a relatively cheap device, kind of radio shacky, but worked within it's limitations. Might have been called an 'active antenna'. Just did a Google search and didn't turn up anything that seemed to be like the one I had. Several homebrew designs and kits but didn't see anything prebuilt except one that was a bit pricey, $400plus.
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Old 25-12-2012, 17:39   #9
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Re: SSB Reciever Antenna Length

Quote:
Originally Posted by SV Demeter View Post
Ok so for example frequency is 8.140MHz so thats 8,140,000Hz? So wavelength is 36.85? Or do I have my decimals screwed up? Also the site referenced give the following options for wavelength:

1/8
1/4
3/8
1/2
5/8
3/4
7/8
Full

So if I'm trying to calculate the optimal antenna length for tuning to 8.140MHz what parameters should I enter in the calculator?

Thanks.
Yes, you correctly calculated the free-space wavelength of 8.140 MHz. There's a catch though: physical antennas shouldn't be cut to the free-space length (or in this case, 1/4 of that length), due to us not living in a vacuum, and the fact that wires have finite thickness. Here's an equation that should get you close for a quarter-wave vertical:

Length in meters = 72 / (freq in MHz). With this equation, an 8.14 MHz whip should be 8.845 meters.

If you just cut a wire to this length and plug it into your radio, I doubt if you will notice any difference. Here's how it might possibly help:

Run a length of 50 Ohm (or 75 Ohm, it doesn't matter much) coax from your radio to somewhere far away from your on-board electronics, and close to a grounding point (chainplate, toerail, wire lifeline, a piece of bare wire tossed into the ocean, etc.) Connect the vertical to the end of the coax (center conductor), and your ground to the coax shield. This is your new tuned antenna, and it might at least reduce the pickup from your on-board noise sources. At the SSB frequencies we're discussing, the coax loss isn't going to be a big issue, so you can use something small and cheap.

If you're interested, a perfect 1/4W vertical does not show your coax a 50 Ohm impedance -- it should be much lower. In practice, ground loss and other factors raise the impedance, so the coax won't be horribly mismatched. You wouldn't want to use this setup for a transmitter, at least not without some fine-tuning and matching, but for the receiver it's OK.

Your receiver's antenna jack probably isn't 50 Ohm anyway. It might respond better to a half-wave vertical. As long as you are hearing atmospheric noise (that extra hiss when you attach the antenna), your antenna is probably good enough.

More geek info: The "active antenna tuners" mentioned here can help your radio if it is being overloaded by strong out-of-band signals. The tuner acts as a filter (as well as an amplifier). Some inexpensive receivers are prone to overload. I've used the Kaito receiver and have not noticed overload problems, but I'm in a pretty quiet area.
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