Hi I am trying to simplify my shack and operations. I use a multi element beam for 20m up to 10m but want to have something else for 40m and 80m or even 160m.
Currently I have tried playing with dipoles cut to 40m and 80m and also tried a loaded diploe for 40 & 80. These all seem noisy on the band so I have put up a loop around the 4 corners of my QTH plot.
In the rear garden corners I have a steel pole section with a non conductive wire up through the middle of each which allows me to pull up or let down anything attached to the dog bone insulator on the end of the wire, these are at approx 25 feet.
In the front garden corners I have smaller steel poles with plastic inserts in the ends which extend up away from the top metal portion by approx 150mm to stop interaction with the wire loop. These are at approx 15 feet.
The QTH plot is 30 metres front to back and 9 metres wide so the loop of copper flex weave is 78 metres in a rectangle. It is fed off centre on one of the long sides at roof apex height (approx 30 feet) with 450 Ohm ladder line which drops down vertically for 20 feet then runs horizontal for 15 feet then drops vertical agin into the shack.
On receive, the loop is very quiet and signals (when bands are good) are clean and clear to listen to, if fact the difference compared to the dipole experiments is remarkable.
My issue is, when using my antenna analyser (MFJ 269) I have looked directly at the ladder feed point using croc clips out of the n type plug and have the following results:
1.810MHz = 400 Ohms / > 3 SWR
2.000MHz = 200 Ohms / > 3 SWR
3.500MHz = 200 Ohms / > 3 SWR
3.800MHz = 400 Ohms / > 3 SWR
7.000MHz = 200 Ohms / > 3 SWR
7.200MHz = 200 Ohms / > 3 SWR

I tried cutting a diploe for 80M over the weekend with approx 19.3 metres in each leg fed from the same ladder point. The runs in each direction do have to dog leg approx 2 metres from each end.
The results with the same test are as follows:
1.810MHz = 100 Ohms / > 3 SWR
2.000MHz = 90 Ohms / > 3 SWR
3.500MHz = 200 Ohms / > 3 SWR
3.800MHz = 200 Ohms / > 3 SWR
7.000MHz = 200 Ohms / > 3 SWR
7.200MHz = 200 Ohms / > 3 SWR

I am wondering if I am measuring it properly or if the feed line I am using is at fault.

I have recently purchased an auto tuner to allow me to just quickly jump between these lower bands but I was trying to get as close to resonance on say 80m which would allow me to use it on 40m and 160m.

I am rightly or wrongly assuming that the feed line itself is not part of the radiating antenna due to its symmetrical construction but do I need to allow for its physical electrical length in the cut length for the band that I am wanting to favour for resonance?

I would really like to keep the loop for the reasons previously stated and the astetics of the property.

Any pointers would be gratefully received.

thanks in advance
Wayne

40 views but no replies....

Okay, are you after a multiband loop which I presume you are?. A 80m loop will require about 270ft of wire, it should be okay with the length you have which is a bit shorter so a ATU will be required.

If you only want it for 80m (it will need a bit of jigging to get the extra wire in) you could use a length of 75 ohm coax (1/4 wave in length) then 50ohm straight into the shack.

But if you are going to use 450ohm ladder line, you also need a Balun, or are you going straight into the ATU?, I have no knowledge of auto ATUs as I have never used one. As in does the ATU you have a built in Balun.

Keith G0TSH

know what you mean


on another note dont you need a 4:1 balun?
thks

Wayne where does the feeder goto when its in the shack eg balun , also remember when using feeder keep away from 1/4 wl as it transforms the impedance .

How are you measuring those figures? With the analyser right at the loop or dipole feedpoint, and with the ladderline disconnected?

Steve G3TXQ

Thanks for your replies everybody.
I have been using it on a balanced (twin roller inductor) tuner with capacitor connected across. This has been tuning ok but it is so slow to change bands.
I do want to use it multi band but as resonant as possible on 80m.
my idea was to attach a toroidal ferrite current balun at the ladder feed point and use an auto atu to tune with a short length of coaxial from the output of the atu to the balun.
I carried out these measurements from the ladder feed point with antenna connected to see what the impedance was over to 80m up to 10m value would be, then to choose an appropriate ratio for the current balun required.
Should I be discarding the ladder from the equation? and measuring at the feed point of the sky loop?
Thanks
Wayne

I think you are worrying too much about getting the loop resonant - there's nothing "special" about a resonant antenna.

Even if you managed to get the loop resonant on 80m, if you then feed it with commercial window-line the SWR(50) back at the shack could be anything from 2.4:1 to 27:1 depending on the length of the feedline.

Putting a remote tuner at (or close to) the loop feedpoint would work. You'll need a 1:1 current balun between it and the loop.

Steve G3TXQ

Hi Steve
OK thanks for your time and comments. I will put the loop back up now and feed it as before. The coax run will only be approx 2-3 meters.
Regards
Wayne.

Hi Steve
I have moved the feed point over the top of the shack now and checked the measurements again at the feed point, the loop is matched at 3.8MHz and shows a fairly constant 100 ohms. I have replaced the feedline for 300 Ohms now and it drops down approx 4 metres then gently right angles into the shack onto the balan.
Do you think the 4:1 is still ok or should it be a 2:1 ratio ?
Thanks again
Wayne

I've forgotten - do you intend to use a tuner with this system?

Steve G3TXQ

Hi Steve
Yes I want to use and auto tuner
Wayne

Then my first choice would be a 1:1 current balun.

Steve G3TXQ

Hi Steve
OK thanks I will just wind one up and test.
Wayne

Hi Steve
Thank you very much to you and everyone for your help. The loop is now working well. My guess is that it must have been the poor way in which I had run the 450 ladder down from the feed point.
Wayne