Shortwave antenna

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alfaeire
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Shortwave antenna

Post by alfaeire » Mon Aug 30, 2021 10:32 pm

Howdy all, I'm looking for broadcast antennas for shortwave, I'm aware of a center fed dipole wire, but I'm wondering is there half or quarter wave antennas that can be used? I'm aware because of the radiating pattern that uprights cannot be used. I'm just short on space and distance right now and I want to play around with the 15w I have.

Can anyone suggest some antenna designs?

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Re: Shortwave antenna

Post by reverend » Tue Aug 31, 2021 9:18 am

If you're really stuck for space and can't get a dipole up, you could use a vertical with a decent ground-plane (chicken wire spread out over a garden would be enough) but depending which band you're using, the vertical might need to be very long (at ~6.4 MHz it's about 12 metres). You could form the vertical into an inverted 'L' shape, or throw it over a roof-top. Also it doesn't have to be fully vertical, it could slope up to a tree, or a building.

It is possible to use a loaded vertical (something like this https://moonrakeronline.com/ampro-40-7m ... le-antenna, which if you extend the top whip section would resonate at ~6 MHz). You still need a decent ground-plane and the feed impedance is low, so for best efficiency you would need a tuner. Efficiency is also not so good so with a poor antenna and 15 Watts, you're going to struggle a little.

The best bet if you don't have the space at your preferred locale is to find a nearby woods and string an antenna across a couple of trees (and take a battery to power the rig). Was it Atlantis or Krypton back in the day who use to send a QSL card which said 'TX site: Small woods').

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Re: Shortwave antenna

Post by Gigahertz » Tue Aug 31, 2021 10:31 am

When I was testing a few rigs on shortwave at my old place I only had a 50ft garden so I used an Inverted-L.

At the furthest point of the garden I placed a 5ft foot ground stake then with a 1/4wave piece of wire for the frequency I operated on it went vertical for 6m then the rest of it horizontal towards the house.

Wasn't the greatest of antennas but with a 10w rig it was heard in a number of European countries. It was also heard faintly in the East coast of the states got sent a mp3 recording of it.

Just careful where you operate and don't use 6312KHz or anywhere close to it!

Also I spoke with someone who used a homemade magnectic loop indoors and was heard in Ukraine on 48M band

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Re: RE: Re: Shortwave antenna

Post by alfaeire » Tue Aug 31, 2021 3:37 pm

Gigahertz wrote:When I was testing a few rigs on shortwave at my old place I only had a 50ft garden so I used an Inverted-L.

At the furthest point of the garden I placed a 5ft foot ground stake then with a 1/4wave piece of wire for the frequency I operated on it went vertical for 6m then the rest of it horizontal towards the house.

Wasn't the greatest of antennas but with a 10w rig it was heard in a number of European countries. It was also heard faintly in the East coast of the states got sent a mp3 recording of it.

Just careful where you operate and don't use 6312KHz or anywhere close to it!

Also I spoke with someone who used a homemade magnectic loop indoors and was heard in Ukraine on 48M band
Have you some photos of it by any chance?

My problem is, I'm in an old apartment building, just moved to northren Europe, theres no roof access and I'm on the 4th (top) floor.

It's probably the least likely location to be able to put it together.

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Re: Shortwave antenna

Post by SamTheDog » Tue Aug 31, 2021 5:08 pm

Hi.
I have started using home made magnetic loops.. I first made one for the 48 metre band and then for 42/43metres. High voltage capacitors are home made too using thick-copper, copper-clad fibreglass. machined on my milling machine and fitted in a waterproof box.

Capacitor is a compromise as it had to be very cheap / disposable. But on the good-side I'm not changing frequency too often so it doesn't need to be motorised or anything...

Going to build another two of them this year, another for 48M and another for a higher frequency rig I have....

I've also used magnetic loops on VHF (band II) and they are tiny....

Anyone here used one on mediumwave? It will certainly be smaller than 75M of steel cables and a helium baloon....
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And many moons ago.. WNKR on FM and Shortwave.
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Re: Shortwave antenna

Post by Gigahertz » Tue Aug 31, 2021 5:10 pm

I didn't take any pictures as I made it hard to see.

The basis of it was taken from ham's website. I've amended the picture of it to give you an idea.

Another option you might want to try is a halfwave endfed. You will either need to build or buy a matching transformer and just add a 1/2wave for the frez. How well it will operate I don't know as I built the transformer but never put it on air yet.
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Re: Shortwave antenna

Post by Gigahertz » Tue Aug 31, 2021 5:18 pm

SamTheDog wrote: Tue Aug 31, 2021 5:08 pm Hi.
I have started using home made magnetic loops.. I first made one for the 48 metre band and then for 42/43metres. High voltage capacitors are home made too using thick-copper, copper-clad fibreglass. machined on my milling machine and fitted in a waterproof box.

Capacitor is a compromise as it had to be very cheap / disposable. But on the good-side I'm not changing frequency too often so it doesn't need to be motorised or anything...

Going to build another two of them this year, another for 48M and another for a higher frequency rig I have....

I've also used magnetic loops on VHF (band II) and they are tiny....

Anyone here used one on mediumwave? It will certainly be smaller than 75M of steel cables and a helium baloon....
Had a go at magnetic loop for 27MHz but due to the capacitor I could only run 5watts before it started to arc over.

When I get the chance will try a loop for MW band but I seem to get good results with my loading coil, variometer and 12m mast with 10w

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Re: Shortwave antenna

Post by jvok » Wed Sep 01, 2021 3:19 am

If you don't have room for a full length dipole you could always try an inverted V. Basically a halfwave dipole but with the arms bent diagonally. Think like old TV rabbit ears but upside down.

If you're in a 4th floor flat you could try an upside down end fed dipole. Basically just hang a length of wire out the window. Tie off the bottom end to a (plastic) drain pipe or whatever so it's insulated from the ground and no one can touch it. 4th floor probably gives you 12m or so to the ground so it will be near enough half wave on the 25m band. You can get away with less than half wavelength if you have an antenna tuner to match it with. For an end-fed you'll also need to make or buy a matching transformer (google "end fed 1:49 transformer").

Verticals can be easily hidden but the problem is you'll always need a good ground system. That means long radial wires and a big enough lawn to bury them in. Ideally radials should be a quarter wave long heading out in all directions. So for 48m band you need 24x24m of land with the aerial somewhere close to the middle, at which point you might as well put up a dipole. You can get away with shorter radials (I do) but the shorter you go the less efficient they'll be.

Try and get yourself a copy of the ARRL antennas handbook (there's pdfs online). If has a whole chapter on compromise antennas. Hams have been dealing with this problem for years and have come up with some pretty clever tricks to make it work.

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Re: Shortwave antenna

Post by jvok » Wed Sep 01, 2021 3:30 am

Anyone here used one on mediumwave? It will certainly be smaller than 75M of steel cables and a helium baloon....
I've been thinking about building a horizontal loop for medium wave strung across the fence tops. I've got a 10x10m garden so would be 40m circumference, should work quite well. Although the maths says high Q might be a problem for full bandwidth audio.

Tbh though with the huge amount of disused former AM sites that have been abandoned in place over the last few years, why build your own? :whistle

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Re: Shortwave antenna

Post by SamTheDog » Wed Sep 01, 2021 1:38 pm

Used horizontal magnetic loops on both 48M and band-II, and a big loop and ATU on Mediumwave, But not a mag-loop on MW

My tests with VHF mag-loops are in attached pictures..
OK for low power, but the capacitors aren't rated high enough for serious power....
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Re: RE: Re: Shortwave antenna

Post by SamTheDog » Wed Sep 01, 2021 3:39 pm

alfaeire wrote: Tue Aug 31, 2021 3:37 pm
My problem is, I'm in an old apartment building, just moved to northren Europe, theres no roof access and I'm on the 4th (top) floor.
Do you have a balcony? If so you may well have space for a magnetic-loop.

I've been experimenting with small transmitting loops with a diameter of between 12 and 16 feet (thats 4 feet a side if you build it 4-sided rather than round!

They are very narrow-bandwidth, Directional if installed vertically, but compact / easy to store and transport
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And many moons ago.. WNKR on FM and Shortwave.
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Re: RE: Re: RE: Re: Shortwave antenna

Post by alfaeire » Wed Sep 01, 2021 7:14 pm

SamTheDog wrote:
alfaeire wrote: Tue Aug 31, 2021 3:37 pm
My problem is, I'm in an old apartment building, just moved to northren Europe, theres no roof access and I'm on the 4th (top) floor.
Do you have a balcony? If so you may well have space for a magnetic-loop.

I've been experimenting with small transmitting loops with a diameter of between 12 and 16 feet (thats 4 feet a side if you build it 4-sided rather than round!

They are very narrow-bandwidth, Directional if installed vertically, but compact / easy to store and transport
I don't
But if you wouldn't mind, send on some details of the build and I can assess what my best plan of action is.

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Re: Shortwave antenna

Post by Albert H » Thu Sep 02, 2021 4:23 am

Does your building have a lightning rod? If it does, you have half of an inverted "V"! Ground the rig to the lightning conductor, and the "hot" to a piece of wire hanging from a window at 15 to 20 degrees from the vertical. Tie the wire off to a tree or lap post with fishing line. and get it high enough so that nobody can touch it (the far end will be the high voltage anti-node!). You'll probably need an ATU for best results, but you should be able top get some serious range with 15 Watts.

Just for info - we used to use this type of aerial off the top of tower blocks on medium wave. One site we used (a block that was in Hackney, but has since been demolished) really got out well with just 8 Watts carrier, 30 Watts peak. There was a listenable signal in most of the home counties in the daytime (it disappeared amidst all the European stuff at night). We made the mistake of putting it on 576 kHz - a clear frequency, but very close to the frequency at which a VHS video recorder puts its colour information. Everyone in the block got psychedelic video recordings!
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Re: RE: Re: RE: Re: Shortwave antenna

Post by SamTheDog » Thu Sep 02, 2021 7:03 am

alfaeire wrote: Wed Sep 01, 2021 7:14 pm I don't
But if you wouldn't mind, send on some details of the build and I can assess what my best plan of action is.
No Problem, I'll dig out a few photo's and pop them up here.....
Sam The Dog.....

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And many moons ago.. WNKR on FM and Shortwave.
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Re: RE: Re: Shortwave antenna

Post by alfaeire » Thu Sep 02, 2021 1:32 pm

Albert H wrote:Does your building have a lightning rod? If it does, you have half of an inverted "V"! Ground the rig to the lightning conductor, and the "hot" to a piece of wire hanging from a window at 15 to 20 degrees from the vertical. Tie the wire off to a tree or lap post with fishing line. and get it high enough so that nobody can touch it (the far end will be the high voltage anti-node!). You'll probably need an ATU for best results, but you should be able top get some serious range with 15 Watts.

Just for info - we used to use this type of aerial off the top of tower blocks on medium wave. One site we used (a block that was in Hackney, but has since been demolished) really got out well with just 8 Watts carrier, 30 Watts peak. There was a listenable signal in most of the home counties in the daytime (it disappeared amidst all the European stuff at night). We made the mistake of putting it on 576 kHz - a clear frequency, but very close to the frequency at which a VHS video recorder puts its colour information. Everyone in the block got psychedelic video recordings!
No lightning rod I'm afraid, but I'm the top / 4th floor, and no access to the roof without scaffold

It's a 48v rig, so I won't be pulling a bank of batteries anywhere.. I think I'll have to have a look around at other locations. There's a large youth center not too far away that I'm involved with, I can perhaps get the kids involved in a radio project, and it has a long felt roof, ajoining building is about 10 to 15 foot taller.

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