HF antennas

[Andrew_B]

I have just bought myself a Radioshack DX394 receiver which I am trying to setup at home before I install the set in the boat.
I am told that the best antenna to use is just a length of wire strung up outside.
Does it matter how long the wire is or is it just the longer the better.Also does it matter is the insulation is still on the wire or should it be removed.

The set has to be grounded - is the keel good for this?

When I permenantly install the set on the boat is the best antenna an insulated backstay.If so what is involved in insulating it - I guess it has to be cut?

HF radios are a completely new world to me so any advice or tips would be much appreciated.

PS If I do manage to set this thing up can someone give me a frequency to tune into to test the signal.


Andrew

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[jimi]

Hi Andrew the plural of antenna is antennae .. hope that helps ;-)

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[Roberto]

Hello Andrew,

<<Does it matter how long the wire is or is it just the longer the better.>>

Usually the longer the better, but with a long aerial you might also pick up too many static noises and parasites so it is worth testing. Try with several meters of a thin common copper wire, if you get too much background noise (esp at low frequencies) then cut away a half meter at a time and see if things improve.

<<Also does it matter is the insulation is still on the wire or should it be removed.>>

you can leave the insulation on the wire

<<The set has to be grounded - is the keel good for this?>>

yes

<<When I permenantly install the set on the boat is the best antenna an insulated backstay.If so what is involved in insulating it - I guess it has to be cut?>>

your set is a receiver, so you do not need an insulated backstay (which is commonly used in emitters receivers)

Also, it depends on what kind of material your boat is built, with a plastic one the wire may not need to be outside, you can fit it under the coachroof lining for example, no need to be perfectly straight.

hope this helps

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[AndrewB]

Back-stay antenna

HF/SSB radio is a world in itself, every bit as complex as sailing! There are whole <A target="_blank" HREF=http://groups.yahoo.com/group/RADIOSHACKDX394/>USER GROUPS</A> dedicated to popular models.

You are right that the most satisfactory thing to use aboard is an insulated backstay. Any good rigging shop will make up one up for you, its not a DIY job, the insulators have to be failsafe. (I had one made by Jimmy Green, who advertise in the yachting press).

Longer is better, but in theory, best reception is achieved by matching antenna length to the frequency. That is obviously impractical, but there is a thing called a longwire balun which compensates, and I found improved reception. Search Google for suppliers, about £25.

You need to earth to some metal in contact with the sea. The propshaft is often used for this.

 

[Oldhand]

Not in the RF community who unfortunately are Amercanised, Antennas is the plural used.

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[Oldhand]

Not in the RF community who unfortunately are Americanised, Antennas is the plural used.

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[Oldhand]

Andrew,

As suggested by one earlier reply, an insulated back-stay makes the best HF antenna on a yacht and a Balun (balanced to unbalanced transformer) should be used with a coax cable between this and the receiver. If the antenna feed cable to the backstay just above the lower insulator is of heavy guage insulated copper wire, then at some future date it could be used for HF transmission as well as reception, should one want to go down that route.

It is best to use a specific earthing plate on the outer hul which is not used for any other bonding purpose, this minimises the possibility of interference from onboard sources. There ar specific "sponge" copper plates for the purpose which present a very high surface area and are again suitable should one want to install a transmitter at a later date.

Testing reception depends on propagations conditions which vary with time of day. Generally, for a given propagation path lower frequency bands work at night and successively higher bands as day time progresses. It would probably be best for you to search the internet for the frequencies of a station you are interested in receiving - you must have bought the receiver with something in mind! If you connected a PC with weatherfax software you could try out Northwood on 2618.5 MHz, 4610MHz or 8040Mhz.

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[Bergman]

Antenna:

Basically longer the better, ideally lengths of odd number of quarter wavelength (300/freq MHZ = wavelength) on the frequency in use. Clearly without elastic wire this is difficult. Best solution is a tuning unit. Receive only units are very cheap, about £25 at Maplins just twiddle for best signal.

Don't know what the "balun" you referred to is, normally a balun is only used in a dipole antenna to connect a balanced source to an unbalanced load. with a length of wire (or backstay) both load (radio) and source (antenna) are unbalanced so not necessary.

Insulation is not important, leave it on, what is important is that antenna is as far away from obstructions and solid objects as possible. For example if using a tree as a support have a length of rope between tree and wire. Same for house, telegraph pole skyhook etc.

When installing in boat I wouldn't use insulated backstay, just run a length of wire up the mast. If you have a ketch go up the mizzen and across to the main, a horizontal bit has advantages. Use the keel as the earth, or any thro hull metal thing, there is no measurable current so no electrolysis problem on receive. (think again if you get a transmitter).

If you look at www.dxing.com/tuning.htm there is a full list of all frequencies from 150kHz to 30 Mhz. In general terms in daylight anything from 20-25 MHz downwards after dark around 10-12 MHz downwards.
10MHZ and 5Mhz (exactly) has WWV time and freq standard,
5. 4 - 5.8 aeronautical service
7.3 to 8.0 has broadcast stations (this is best to check out your gear)
14 - 14.35 should have amateurs from europe and USA during morning and early afternoon.

Hope this helps



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[Birdseye]

Assuming that your set is receive only, then you do not need to put insulators (which are expensive) in the backstay, and which must to some degree weaken it. Much better to get yourself a good length of plastic insulated half decent multistrand copper and run it up the mast on a halliard when you want to listen.
For a receiver, you dont need really heavy wire because the currents are tiny, and you dont need it erected all the time because its not a safety system. Thats not to say that all the above advice isnt good and technically sound, but I question whether you need to go that far when all you want to do is listen to the nets. If you subsequently find that what you receive is interesting, then you may want to do a pukka job. But I wouldnt suggest it to start

Ground on your anode, but I suspect the radio will work quite well without grounding at all. Try it.

Make sure the battery feed is well suppressed. Might be worth getting some ferrites from Maplins and putting them round the feed wires to the radio.

The alternative to all this is to get a simple active aerial (ie one that contains a small amplifier) and stick it on the taffrail. That's what NASA do with their HF weather receivers, and I found that it worked almost as well as the backstay I use for my ham radio transmitter.

And "antenna" is something on an insect, or a nasty Americanism for the aerial attached to your wireless.

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[Bergman]

Agree about the backstay - thought that was what I had said.
Agree about ferrites on power lead - anything that reduces noise is good.

Which is why I disagree about active aerial - my experience is that they amplify noise and pick up little signal which is not good.

Similarly with thin wire, certainly the currents are small but are just as subject to Ohms Law as any other currents. Within reason thicker the better.

Again with omitting earth, yes the receiver will work but remember this receiver will almost certainly have little or no RF selectivity and so will have no means of rejecting any noise or strong signals being presented to the mixer. This will almost certainly significantly impair the signal/noise ratio which is what we need to consider. That is why I strongly recommend an ATU, it effectively gives you a significant degree of front end selectivity which improves the S/N ratio and very much improves rejection of strong (unwanted) signals. Clearly if an ATU is being used it can only tune the aerial effectively if there is an earth for it to tune against since the earth is effectively one part of the aerial system (with a Marconi aerial)

I do not see any role for a balun in this system - there is no balanced component so no unbalanced to balanced match is required. The ATU will give an impedance match to the unbalanced single wire feed to the aerial which tunes against earth.



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[Birdseye]

Andrew - keep it simple and cheap until you find out if there is enough on the airwaves to be worth going further. We ended up just using the receiver for the world service, and a 20ft length of wire in the cabin will work for that - assuming you dont have a metal boat.

If you find the nets inteesting, then go for a ham license and a transmitter. That will definitely require a proper aerial, and an atu.




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[milltech]

Nasa do a neat little active HF antenna to work with their HF/3 models, and which can be coachroof or rail mounted. I have no idea if it would work with anyone else's radio but you might ask 01438 354033 or nasamarine@aol.com.

Nasa's MD is first and foremost a "radio" man.


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