Home made antenna for VHF

[majoco]

You could modify your transformer - cut all the teeny little wires and throw them out. Get two lengths of about a couple if inches of thin wire - I strip old printer cables - about 24 wires of some lovely colours - I'm assuming you have a little binocular core. Take one bit of wire and go in one hole - turn around and come out the other - now go back in the first hole and out the other. Equalise the lengths so that the ends sticking out are about the same length - you now have a two-turn winding. Repeat for the other bit of wire except go in from the other side. Solder one winding to the the dipole elements and the other to your coax cable - shorten the ends as much as you can. Now you have the 72 ohm impedance of the dipole well matched to the 75 ohms of the cable.

 

[Ubbe]

The transformer impedance mismatch will give a 2dB loss but you also are using the additional signal from the lower element, the one that would had been connected to the coax braid and only had been working as a groundplane. And the direction loob will be straight out horisontal with the balun instead of screwed up from having some part of the coax working together with the lower element. All in all I think the balun will give the best solution.

I measured a GP antenna for 155Mhz and it has 300 ohm impedance at 125Mhz and 180Mhz and would have a perfect match with a 300/75 balun so it will help making the antenna more wide banded, that's the way they do it on the ST2 antenna. For receiving just a narrow band a 1:1 balun would be better and you can actually get one as a sample from Mini-circuits if you promise to give them feedback from your project. They are SMD types and the easiest way to use them are to take a CATV splitter with F connectors and open it up and cut the traces on the circuit board to the connectors and solder wires to the balun.

/Ubbe

 

[majoco]

I thought the whole idea of an OCFD was that it was broad banded with a centre impedance of around 450 ohms - the HF version works very well with a 9:1 transformer.

Ham Radio Site - [Off-Center Fed Dipole]

 

[prcguy]

An offset center fed for HF generally uses a 4:1 transformer and the antenna is matched around the 200 ohm point where it works well across most harmonic multiples of its lowest design frequency. I use one as my main HF antenna at home and its one of the best all around HF wire antennas I've used.

However, I think its a wrong idea to use one for VHF/UHF because on most bands except for the lowest resonant band it will have lobes pointing up and down and everywhere but the horizon where you need it. Bottom line is it will have loss at the horizon on most bands compared to a dipole or ground plane.

I thought the whole idea of an OCFD was that it was broad banded with a centre impedance of around 450 ohms - the HF version works very well with a 9:1 transformer.

Ham Radio Site - [Off-Center Fed Dipole]

 

[bchappuie]

Way too much here.///// 18" piece of #12 copper soldered to the center of an SO-239 connector mounted on a 6" square piece of flashing, feed RG58 off of that to the scanner.

 

[cmdrwill]

1/4 Wave Ground Plane Antenna Calculator

Very simple to build and works with 50 ohm RG58 cable.