homebrew aerial

humaxf1

Im trying to cut down on the noise for HF RX (80m thru 20m). At the moment, I'm using about 70ft of wire into a MFJ 941 tuner.

As an experiment I rustled a 4:1 balun together to use as a dipole center. I've attached a few pics if anyone has any comments or ideas to improve it. Just in the experimental stages so not worried about the weather-proofing aspect yet.

3x T200-2 toroid cores for $23 incl post

S0239...free

Wire...free

Plywood...free

The toroid is hot glued in 4 spots. All I need to do now is drill for bolts to acts as terminals and maybe a second either side for strain relief and double the wire back on itself.

There are many websites praising the carolina window...approx 40ft and 90 ft for the elements and a coax choke close to the feedpoint for good measure.

Anyone dared to try anything like this? TIA

stylers

Nothing obvious wrong with the construction that I can see, but you might want to keep the weight down, as the balun will be suspended by the radiators, and will also have the coax hanging from it. where did you source the toroids ?.

watty

For handling power I found that an old CRT TV LOPT core (unscrews into a pair of [ and ] halves) is good.

I've also found that losses /power better simply winding on a piece of 2" approx PVC waste pipe

10 turns of 2Amp insulated mains flex on top of 20 turns (That's a 4:1 impedance).

one coil is connected to SHORT coax feed to ATU.
The longer coil connects to 66ft wire (80ft won't tune, next sizes are 33ft or 120ft), with far end insulated with nylon or polypropylene rope about 4ft long minimum.

The other end of that coil is connected to a vertical wire going to a 4ft long ground spike in a wet flowerbed. (Under path needs a longer spike as it's drier).

humaxf1

Thanks guys...

stylers: Never thought about the weight aspect...well spotted.
I sourced the toroids from http://www.kitsandparts.com/ You name it, they got it.

Details here for winding, http://www.pd7bz.com/radioprogs/manuals/balun_transformer.htm I used the T200-2 toroid with 17 turns. It's an easy to work with size.

Watty: Been looking at the air-wound baluns too. http://rogertango.com/articleread.aspx?idnumber=34232411 Just to confirm, just one end (or both) of the 10turn coil connected to coax centre AND braid? Other part sounds straightforward enough. Suspend as high as you can? so vertical wire length not important?

Thanks for the comments...

stylers

@ watty,

I've never tried using LOPT cores for HF work, I would have thought they would be too lossy at HF but i suppose it depends on the grade of ferrite used. I have a box full of toroids and cores pulled out of stuff but alas its impossible to know what they will work at..

watty

PSU filter toriods are worse as they are meant to be lossy!

LOPT has to cope with huge energy of flyback edge. The Line Frequency may only be 15KHz, but you need many Megahertz low loss or the dot can't get back at the end of the scan line. They will handle 200W without getting warm. A regular toriod the size of 1l bottle cap can shatter at that power if it's the wrong type!

But nowadays I only use aircore. (PVC pipe) Better frequency response, lower loss etc..

Put 5 to 10 turns x 2 on a core as a 1:1 transformer with 10W dummy load and feed about 5W to 10W from a 100W amp. If it warms at all it is too lossy. Test at 1.9, 7, 18 & 28MHz as a minimum. Optionally connect / measure power TX side and Load side. Look at SWR (reverse voltage really) too