10 metre loop antenna.

[slderiron]

Split from this thread:-

https://www.vintage-radio.net/forum/...ad.php?t=71064



Right, well, I've had a think about this over the last few days, and given the advice given and further research on the web, I've come to the conclusion that a 5/8 wave vertical would be a waste of money.

I bought a length of copper pipe for £14 and repaired my half-wave. It seems to be working better than before, perhaps its the properties of the copper or maybe I've tweaked something unbeknown.

I've decided to build a loop. Now, this is quite new to me, I've never built an antenna before. I found some excellent instructions here
www.m0ysu.com/index.html its for 11m but easily done for 10m, I have the measurements. But there's just one thing I'm a little stuck on. The choke balun. He states that at the point of the 50ohm coax joining the 75ohm coax he makes a choke balun with 7 turns. What would the internal diameter of the windings be on the 50ohm coax?

I ask this because I don't want to wind it too tight for fear of splitting the outer braid, but want to ensure the windings are tight enough.
Regards, john.

[GMB]

I haven't read the article but from what you say...

A choke balun is just a bit of inductance common to both inner and outer. You can make it any way you like. More turns will work better, bigger turns will work better. It's a compromise anyway. So changing from 11m to 10m I would say - just do it the same.

[Tony1951]

Originally Posted by slderiron

But there's just one thing I'm a little stuck on. The choke balun. He states that at the point of the 50ohm coax joining the 75ohm coax he makes a choke balun with 7 turns. What would the internal diameter of the windings be on the 50ohm coax?

Hi John,

You can do it like this:

https://picasaweb.google.com/Volpoon...47973400870034

This was an experiment I made about two months ago. It is still up there and it works, but isn't that great. I have two attempts at choke baluns on it. These stop the feeder acting like a part of the antenna and radiating common mode currents. They are also supposed to cut noise pick up.

The first one was simply a bunch of coils bound together with tape, but I thought it too small an inductance to do anything much, so I made another with some twin 30 amp power cord wound on a ferrite rod. NOW I KNOW that a ferrite rod is supposed to saturate and waste power, but I ran a full ten minutes of 100 watt high duty cycle PSK into that antenna and went up and took hold of the ferrite choke balun in my hand while it was still transmitting and it wasn't in the lest bit warm. Since it didn't heat up, I conclude that it wastes nothing, else where would all the power go that was being wasted?

This antenna is made of twin lead and is acting as a folded dipole which is open at the far ends. A folded dipole normally has a feedpoint impedance of 300 ohms, but because this one is folded into a loop as well, this is reduced to 50 ohms, so I feed it with a length of coax and no atu. When I first made it to the correct length, the swr was about 2:1, so I simply folded up short lengths at the ends as you can see in the photos where the string is, and it rapidly reduced to 1.2:1 or 1.3:1, pretty much right across the 20 meter band.

Notwithstanding what i have said there, this small loop is not a patch on the new one which is outside, much bigger and made of FAR thinner wire. I think that part of the excellent performance of my 40 meter full wave loop on 20 meters is down to the sheer amount of wire in it (total cost of 2 bobbins of 26 swg wire less than £5. If you have the space, do try it, or maybe a twenty meter loop outside and use it on ten. Mind you, I don't know what you will find to work on ten most days, unless it is a local net of hams who like to chat.


This link is what I used to make my big antenna, but mine is only 145 meters right around. This link is well worth a read even though the guy is talking about a top band full wave loop with five hundred and eighty feet of wire in it:

http://www.k5rcd.org/hor%20loop%20instruct.htm



Mine is a full wavelength at 7.1MHZ and it is only about 17 meters long at its longest length. A full wave loop at 20 meters would only be half that size and made of thin wire is really not visible if you have trees at the bottom of your garden and attach the bottom corners to poles shoved up in the foliage.

This photo shows how hard it is to see the actual loop, albeit that you can easily see the pole it is attached to.

https://lh5.googleusercontent.com/-6...0/P5050001.JPG

https://lh4.googleusercontent.com/-r...0/P5050002.JPG

[MichaelR]

Tony,

You have inspired me to try a loop. I should be able to get over 200 feet of wire up.

Surely any antenna that can radiate at a height of 20 metres is going to perform reasonably well have you tried the loop at lower heights ?.

Mike

[slderiron]

Yes I see Tony, so it doesn't have to be wound very tightly, large enough to place your hand through. Just some gaffer tape to keep it together. I don't think I'll need a ferrite rod, several windings of the 50ohm coax should be enough.

This has been very helpful Tony. I have an antenna rotator I bought second hand about 3-4 years ago but never got round to using, and some of the other parts, so I think I could get this thing going fairly cheaply.

Best regards, john.

[Tony1951]

Originally Posted by MichaelR

Tony,

You have inspired me to try a loop. I should be able to get over 200 feet of wire up.

Surely any antenna that can radiate at a height of 20 metres is going to perform reasonably well have you tried the loop at lower heights ?.

Mike

Hi - sorry I just saw your question. Haven't been back for a couple of days Mike.

The articles that I read on the interweb all said that a full wave loop will operate really well at a pretty low height. Not wishing to outrage the neighbours, I am, like most of us restricted to fairly lowish height antennas - far less than the optimum for HF. I can't really get any of my wires above the height of the roof of the semi that I live in, and down the garden, it is less - maybe 18 - 21 feet at the end of the loop. The guy who wrote the 'skywire' article referred to in my earlier post was talking about something like twenty feet, I think as I recall.

Another great thing about this antenna is that it is omnidirectional, unlike a vertically oriented loop. If you ever want to check how my antenna is working, go to the page linked to below and enter the details for my callsign, G0BZB, the 20 meter band, and a time period of say 6 hours and you will see where I am being heard in that time period. You can enter a lot of parameters in there like different time periods and other things. Likewise on the 'Database' page. that one shows you the automatically reported signal strength of my signals expressed in SNR (signal to noise ratio). Obviously, since this is a weak signal mode and since I never use more than five watts and sometimes operate with 100 milliwatts, the reported signals are well below the noise floor, but because WSPR is a clever mode with very long symbol duration, any old PC with a sound card connected to a radio transceiver, can spot the coherent feeble signal well below all the random spikes of very short duration radio noise. Hence, with less power than you wold use to light a tiny torch (100mW) I was heard in Australia and all over the States.

Here is the link to one of the most fascinating aspects of the hobby I have seen in thirty years of Ham Radio. (Excuse the blatant advert for the mode).

http://wsprnet.org/drupal/wsprnet/map

[Tony1951]

Originally Posted by slderiron

Yes I see Tony, so it doesn't have to be wound very tightly, large enough to place your hand through. Just some gaffer tape to keep it together. I don't think I'll need a ferrite rod, several windings of the 50ohm coax should be enough.

This has been very helpful Tony. I have an antenna rotator I bought second hand about 3-4 years ago but never got round to using, and some of the other parts, so I think I could get this thing going fairly cheaply.

Best regards, john.

Hi John,

A horizontal loop is omnidirectional in its radiation so you won't need to rotate it. In effect it is like the VHF / UHF 'Halo' antenna. So just stick it up and power it up and it will work fine. I think there is definitely something in the idea that the bigger the loop the bigger the signal. It stands to reason that the more high current nodes there are along a wire the more signal you will radiate. Take a half wave dipole as an example, and think if the representations of current and voltage peaks along its length from the old RAE days when we were learning the theory for an amateur radio licence. The diagram I am thinking of shows that there is a current high point at the centre of a resonant half wave antenna, and the theory tells us that this is where the radiation happens, not at the ends where the voltage is high and the current is low. Well, my full wave forty meter loop has four times the number of high current points along it than a twenty meter dipole when energised by a twenty meter RF source. That is why I think it is now beating the pants of my mates excellent twenty meter sloper that used to beat my old W3DZZ into a cocked hat. As I said before as well, it is also omni directional unlike a dipole and most other HF antennas. When I say it is beating my mate's antenna, I have objective evidence in the form of SNR of both antennas. Because we can check the reports on teh internet in more or less real time (they come back about twenty seconds after you make a transmission and can be read on the WSPR web site) we can compare how a transmission from each of us made at exactly the same time was received at a particular location thousands of miles away. ALL WSPR TX and RX sessions are timed exactly to happen at the same instant on two minute periods, and when our stations are taking a turn to receive, they can receive multiple transmissions simultaneously, so my signals and my mates ought to appear in the same station's log at the WSPR website, and because we live in the same town three miles apart and the ionosphere was identical for both of us at that time, it is a fair test of our antenna's. My w3dzz used to be about 3db below the performance of my pal's sloping 20 meter dipole, now I am typically 3 to 6 db ahead of him with the new 40 meter full wave loop. In one or two directions he beats me slightly, because of the directivity of the dipole, but I also get far more reports from stations that don't hear him at all.

regards

G0BZB - op Tony

These days getting power out isn't the problem (unless you are lucky and live in the middle of a field!) it's getting a useable signal back, the problem is noise from SMPS, CFL lamps PLT etc. get reception right first. For example my transmit antenna is a 10m vertical which is absolutley dreadfull on receive, for that I have a Welbook loop at the end of the garden as far away from other houses as possible. With this setup I can get 'round the world' on 5 watts.

"If you can't hear them, you can't work them"

[Tony1951]

Originally Posted by merlinmaxwell

These days getting power out isn't the problem (unless you are lucky and live in the middle of a field!) it's getting a useable signal back, the problem is noise from SMPS, CFL lamps PLT etc. get reception right first. For example my transmit antenna is a 10m vertical which is absolutley dreadfull on receive, for that I have a Welbook loop at the end of the garden as far away from other houses as possible. With this setup I can get 'round the world' on 5 watts.

"If you can't hear them, you can't work them"

I could not agree more with that. I have had problems with noise from next door that drove me out to working 40 and 80 meter mobile parked up. It was a revelation how quiet the bands actually are when you are ten miles outside the city. Last winter, I was getting s8 and s9 noise on 80 meters, and not much better this year. They have an array of noise making stuff next door including plasma TV and PLT networking. They can even ruin the Medium Wave band for broadcast signals.

However, that was why I tried WSPR. I can hear 5 watt signals (or my computer can) from the opposite side of the planet. The loop is also an inherently noise rejecting antenna, though I can't explain the physics of it.

[Dave UXB]

Originally Posted by slderiron

But there's just one thing I'm a little stuck on. The choke balun. He states that at the point of the 50ohm coax joining the 75ohm coax he makes a choke balun with 7 turns. What would the internal diameter of the windings be on the 50ohm coax?

Actually, it is not vital. Just wind it neatly and wrap it up so that each turn is tight to it's neighbours. You can use plastic tape or cable ties for this job.

One thing puzzled me about the article: 9 feet is a quarter-wave on 11m. I did not see anything about the quarter-wave being an 'electrical' length (9ft minus vpc), and I suspect that it could be important.

I wonder, though, have you considered a 10m slim jim ? It was in Practical Wireless. And it does not require a ground plane.

[MichaelR]

Originally Posted by Tony1951

Hi - sorry I just saw your question. Haven't been back for a couple of days Mike.


Here is the link to one of the most fascinating aspects of the hobby I have seen in thirty years of Ham Radio. (Excuse the blatant advert for the mode).

http://wsprnet.org/drupal/wsprnet/map

Hi Tony,

I too have been away for a few days so sorry about the delayed reply. OK on the Loop, I am going to give it a go.

The low power work I have never been aware of before so you have taught me something else ..... again fascinating stuff.

Thank you for posting this information

Mike