Antenna Matching

ronv

Well-Known Member

Most Helpful Member

• Feb 23, 2011

• #1

I built this TV re-transmitter. It takes the output from a VCR or such and retransmits it. It works good with the 75 ohm dummy load (good picture and sound, nice 66 Mhz. sine wave output) but when I attach it to a simple dipole antenna the output is distorted and I can't pick up the signal on the TV. I think it's because I can't tune it to the antenna???? Any ideas.

 

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MikeMl

Well-Known Member

Most Helpful Member

• Feb 23, 2011

• #2

More likely, the reflected impedance from the antenna pulls the oscillator off frequency.

 

crutschow

Well-Known Member

Most Helpful Member

• Feb 23, 2011

• #3

What is the length of the dipole? Here's a dipole impedance calulator to help you determine the proper size.

 

nsaspook

Well-Known Member

Most Helpful Member

• Feb 23, 2011

• #4

Small series resistors in the base or collector can help with gain stability. You might just have a parasitic oscillation from the antenna mismatch or from poor shielding.

Page 18:
https://www.electro-tech-online.com/custompdfs/2011/02/ampdesign.pdf

 

4pyros

Well-Known Member

Most Helpful Member

• Feb 23, 2011

• #5

Use 75 ohm coax and do the open air loops as suggested by the link in post #3. Thay well help reduce reflectoins.
Andy

 

ronv

Well-Known Member

Most Helpful Member

• Feb 24, 2011

• #6

Thanks for all the tips!

I think I may not have made the antenna length as colse as I might have - I used 66 Mhz. for channel 4. I should have used 69 for the center of it.
The balun may also help.
I think the layout is okay but I will keep the resistors in my back pocket if the others don't fix it.
I was afraid I had made a basic mistake - maybe it will work after all.