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The big requirement for coax on voltage probes is low capacitance - you generally don't worry about impedance.
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That's because the antenna is largely capacitive (so it looks like a voltage in series with a capacitor), and shunt C at the receiver reduces the signal level (think of it as a capacitive voltage divider). In some ways, this is similar to high performance audio cables with high Z sources and amplifier inputs. -----Original Message-----
From: <[email protected]> Sent: Apr 7, 2025 9:31 AM To: <[email protected]> Subject: Re: [nanovna-users] Dummy Antenna and FM Alignment The high-Z coaxial cable is more for the AM portion of the radio. On AM frequencies, especially the European version, the "stinger" antenna is nothing but a capacitive probe. Consequently, it presents an extremely high impedance. The loss in the 93-ohm line is considerably less at the AM broadcast frequencies than a standard 50-ohm cable would offer. However at FM broadcast frequencies, the external "stinger" antenna is a reasonable portion of a wavelength, although still a bit short. Therefore, it offers something more manageable considering losses. If you're just doing FM alignment, 88 to 108 MHz, a 50-ohm source/load is appropriate. Some older units may have a 300-ohm balanced input. In that case you need a transformer/balun to convert from 300-ohms balanced input to an unbalanced 50-ohm load. That's the purest approach and theoretically correct. HOWEVER, if its 300-ohmsbalanced input, just take a visit to Home Depot, Lowe's or any big-box distributor and buy a cheap 300-ohm to 75-ohm converter and use your 50-ohm equipment. Dave - WØLEV On Mon, Apr 7, 2025 at 2:43?PM AArnaud via groups.io wrote: Hey everyone,-- *Dave - WØLEV* -- Dave - WØLEV |
