How to match TWO 70 ohm dipoles to 75 ohm line? - SOLVED ! -

[K1JJ]

Hola!

[SEE SOLUTION in POST BELOW]

I wonder if someone here knows how to feed/match TWO 70 ohm dipoles to a 75 ohm coaxial feedline using coaxial transformers?

Normally, to feed/match two 50 ohm antennas together for 50 ohm feedline, one would use a 1/4 wave 75 ohm coax length from each antenna tied together at a "T", giving 50 ohms.

In this case, two ~75 ohm dipoles fed with two 125 ohm RG-63 [125 ohm coax] 1/4 wave coax lengths would do about the same thing.  But I have no RG-63/U coax here.

Anyone know how to do it with 35 ohm or 50 ohm or 75 ohm coax transformer sections???  I prefer not to use a 2:1 torroidal unun.

[If anyone has a 10' piece of RG-63/U, please let me know if no one comes up with another solution]   This project is a 6M turnstile antenna... two dipoles at right angles fed 90 degrees out of phase giving an omni pattern.

73,
T

[Glenn K2KL]

I don't suppose RG-62 would help? (92 ohms) I have some of that.

Hola!

I wonder if someone here knows how to feed/match TWO 70 ohm dipoles to a 75 ohm coaxial feedline using coaxial transformers?

Normally, to feed/match two 50 ohm antennas together for 50 ohm feedline, one would use a 1/4 wave 75 ohm coax length from each antenna tied together at a "T", giving 50 ohms.

In this case, two ~75 ohm dipoles fed with two 125 ohm RG-63 [125 ohm coax] 1/4 wave coax lengths would do about the same thing.  But I have no RG-63/U coax here.

Anyone know how to do it with 35 ohm or 50 ohm or 75 ohm coax transformer sections???  I prefer not to use a 2:1 torroidal unun.

[If anyone has a 10' piece of RG-63/U, please let me know if no one comes up with another solution]   This project is a 6M turnstile antenna... two dipoles at right angles fed 90 degrees out of phase giving an omni pattern.

73,
T

[K1JJ]

Thanks for the offer, Glenn.  Though, a friend has given me the solution using 50 ohm and 75 ohm coax as shown below ...

73,
T


* 6M Turnstile Feed Method: *


"Tom,
 
Orient your two 70 ohm dipoles 90 degrees to one another with  their
feedpoints as close together as practical (one mounted slightly above  the other on a common mast).
 
Connect the feedpoints of your two 70 ohm half wave dipoles  together with 1/4 wavelength of 75 ohm coaxial cable (this is the 90 degree  delay line). Then connect 1/4 wavelength of 50 coaxial cable to the bottom  dipole's feedpoint (in parallel with the delay line which is already across its  feedpoint). Install a UHF connector (PL-259) at the other end of this short 50  ohm matching section. Insert a UHF double female for connection to the UHF  connector on your 75 ohm feedline.
 
Remember that energy travels slower in coaxial cable than it  does in free
space so that the above mentioned 1/4 wavelength  sections should be shorter in physical length than what you calculate  (shorter by the coaxial cable's specific velocity of propagation). At 50.125 MHz  you should calculate approximately 59 inches for a free space 1/4 wavelength.  Assuming you were to use simple solid polyethylene dielectric coaxial cable you  would need a physical length of only 67 percent of 59 inches to arrive at 1/4  wavelength."