Transforming 50ohm to 75ohm

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thoughtfulmonkey
Posts: 2
Joined: Fri Aug 19, 2016 9:11 pm

Transforming 50ohm to 75ohm

Post by thoughtfulmonkey » Fri Aug 19, 2016 9:31 pm

Hi,

I've constructed a Quadrifilar Helix Antenna using the calculator on the jcoppens site (https://www.jcoppens.com/ant/qfh/calc.en.php), with the soldered coax connection at the top and wrapped around a few times for a balun. It's the first aerial I've made, and I'm not really sure what I'm doing.

On another site I've read that it's important to use a transformer between the 50ohm cable and the 75ohm SDR dongle, and there was a recommendation for a "twelfth-wave matching transformer".

Basically, my question is can this go on the main tube of the aerial, just after the balun? Does it matter if it is inside the loops? My reasoning is that the soldered connections would be subject to less bending.

Thanks.

rtlsdrblog
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Joined: Mon Nov 19, 2012 11:54 pm

Re: Transforming 50ohm to 75ohm

Post by rtlsdrblog » Sat Aug 20, 2016 9:57 am

There's no need to worry about mismatch between 50 and 75 ohms. The losses from the mismatch are completely negligible. The slight mismatch only really matters if you intend to transmit on the aerial.

thoughtfulmonkey
Posts: 2
Joined: Fri Aug 19, 2016 9:11 pm

Re: Transforming 50ohm to 75ohm

Post by thoughtfulmonkey » Sun Aug 21, 2016 2:27 pm

Thanks for you reply.

I just plugged the 50ohm straight into the dongle, and it's receiving signals. I just need to check that it is tuned to 137.5mhz and can receive NOAA transmissions.

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