need some help making this lead thanks

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prettypolly
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need some help making this lead thanks

Post by prettypolly » 16 Oct 2017, 16:56

Hi can any body please help me with this lead I'm scratching my head here looking very puzzled ..the frequency I've been told its for is 27.465 mhz cheers mike
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Re: need some help making this lead thanks

Post by wa10 » 17 Oct 2017, 00:27

first part is combining two 50ohm antennas in parallel using two 75ohm 1/4wave impedance inverters

pick your frequency & tune both polarities for as close to 50ohm as possible,
voltage & current relative phase will shift if the antennas are not 50ohm

cut two electrical 1/4waves of 75ohm coax using the velocity factor of your 75ohm coax & the frequency you tuned for, measured from tip to tip of pl259's

screw them into a t-piece,

run 50ohm coax from t-piece to rig,


second part is creating a 90 degree phase delay by adding a 1/4wave of 50ohm coax when screwing the harness straight to the antenna,


cut an electrical 1/4wave of 50ohm coax for the same frequency using velocity factor for your 50ohm coax,

connect the 50ohm 1/4wave to one of the 75ohm lines,

connect opposite end of 50ohm line to horizontal on antenna,

connect the other 75ohm line to the vertical on antenna.


if the harness is not been connected right at the antenna, make the 75ohm matching lines & make the coax running to the horizontal an electrical 1/4wave longer than the coax running to the vertical.

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Re: need some help making this lead thanks

Post by paulears » 17 Oct 2017, 08:26

I've never seen a circularly polarised antenna for 27MHz, it must be a bit of a beast! For these to work at the higher frequencies, your soldering and measuring needs to be very precise - at 70cm, for example, we're talking mm accurate, but at 27MHz, at least you have a bit of leeway. Maybe 4 or 5mm I think before the tuning balance goes off. It usually means you need spare cable, because one slip, and you are too short, so need to start again. The diagram just needs you to be able to cut and solder accurately, once you've done the maths. The harness also carries out the function of power splitting, so it's important the two antennas are identical in terms of VSWR and radiation pattern, or the balance is shifted producing unexpected results. If people combine beams, the one that has vertical polarisation is often compromised by the support pole, making it's performance different from the other, so many people then mount the two beams at 45 degrees to the horizontal, so each one has identical performance, looking like an X from the front instead of a +. Try to visualise it as a T shape, with an extra length attached to one of the ends, making it longer, and because it's longer, the signal is also time delayed with the other. The 75 Ohm sections are there to carry out the impedance correction that needs sorting when you combine two 50 Ohm loads.

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prettypolly
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Re: need some help making this lead thanks

Post by prettypolly » 17 Oct 2017, 09:07

its a 6 element maco comet ..and another question can you get 75 0hms coax based on like rg 213 thickness ?

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