Using 75 Ohms TV Cable for Amateur Radio Feedline
Using 75-ohm television cable (like RG-6 or RG-59) for an amateur radio feedline is not only possible but, in certain scenarios, actually advantageous. The amateur radio standard is 50 ohms, but the impedance mismatch is easily manageable. Here is exactly how to integrate 75-ohm coax into a 50-ohm station.
The Mathematics of the Mismatch
When you connect a 50-ohm transmitter to a perfect 50-ohm dummy load using a 75-ohm cable, the resulting Standing Wave Ratio (SWR) is simply the ratio of the impedances:
SWR = 75/50 = 1.5:1
An SWR of 1.5:1 is perfectly acceptable for modern transceivers. At this ratio, only about 4% of your transmitter’s power is reflected back to the radio, meaning 96% of your power still reaches the antenna. A transceiver’s internal automatic antenna tuner (ATU) will instantly flatten a 1.5:1 mismatch, allowing the radio to deliver full output without folding back.
Practical Applications
1. Feeding Half-Wave Dipoles
A resonant half-wave dipole in free space actually has a feedpoint impedance of roughly 73 ohms, not 50 ohms. If you are stringing up a standard wire dipole for the 80m or 160m bands, 75-ohm coax is mathematically a better match at the feedpoint than 50-ohm cable.
Because the antenna’s impedance closely matches the cable’s impedance, the SWR on the feedline remains practically 1:1, and the 1.5:1 mismatch only occurs right at the transmitter, where an ATU handles it easily.
2. VHF/UHF Satellite Operations
High-quality satellite TV RG-6 has excellent shielding and very low loss at VHF and UHF frequencies — often outperforming standard 50-ohm RG-58 over long runs. For LEO satellite work using a Moxon-Yagi hybrid or similar directional antennas, the lower signal loss of RG-6 easily outweighs the minor 1.5:1 SWR penalty.
3. Quarter-Wave Matching Transformers
You can use a specific length of 75-ohm coax to match a 50-ohm feedline to a higher-impedance antenna (like a Delta Loop or a Quad, which sit around 100–120 ohms).
A quarter-wavelength section of transmission line acts as an impedance transformer according to the formula:
Z0 = √(Zin x Zout)
Where Z0 is the 75-ohm coax. If you connect your 50-ohm transmitter (Zin) to one end of a 75-ohm quarter-wave section, the other end will transform the impedance to exactly 112.5 ohms (75 = √(50 x 112.5), providing a perfect match to loop antennas.
The Hardware “Gotchas”
If you decide to use 75-ohm cable, you must navigate a few physical limitations:
- The Aluminum Braid: Most modern RG-6 uses an aluminum foil shield and aluminum braid. You cannot solder aluminum with standard tin-lead solder. Do not attempt to solder a standard PL-259 (UHF connector) directly onto RG-6; the connection will eventually fail or corrode.
- The Connector Solution: The most reliable method is to use a high-quality, weatherproof compression F-connector designed specifically for RG-6. Once the F-connector is properly crimped on, attach a simple F-female to PL-259 (or BNC/SMA) adapter to connect it to your radio or amplifier.
- Power Handling: Standard RG-6 can easily handle a 100W output from a linear amplifier without dielectric breakdown or overheating. However, if you ever plan to run full legal limit (1500W), RG-6 will quickly overheat.