Choosing Coax

The biggest, and most often overlooked factor, is how the antenna is matched/tuned. If it’s a resonant antenna that is better than a 1.5:1 match to the feedline (what your radio sees is irrelevant, it’s the feedline to antenna match that affects efficiency), then the difference between RG-8X and a mid-grade coax like RG-213 is going to be relatively modest for any length of coax less than a couple hundred feet. Likely less than 3dB, so only half a S-unit. Less than 50 feet and you probably won’t notice much.

On the other hand, on 6 metres and up, the difference between RG-8X and the higher end coax like LMR-600 can be quite large, even on relatively short runs with a good match.

100 feet of RG-8X on 2 metres with a 1:1 match will deliver approximately 40% of your power to the antenna. 100 feet of LMR-600 gets 80% to the antenna.

On 70cm, 100 feet of RG-8X at 1:1 SWR gets just 15% of your power to the antenna, while LMR-600 gets nearly 70% to the antenna. THAT is a big difference, and you will likely notice a significant improvement in transmit range.

When the match is poor, such as an antenna that is non-resonant and is tuned at the radio, high quality coax is extremely important. So important, that it is often better to just not use coax at all and instead use balanced line, which generally has about 50% less loss than even LMR-600. At 3:1 SWR, the loss in the line will increase by around 30% (ballpark, it isn’t a perfectly straight line). A 30% increase over 1dB loss is only 1.3dB total loss. A 30% increase over 3dB of loss is 4dB, the worse the coax is to begin with, the bigger the hit.

With regards to receiving, if you have a decent quality receiver (this excludes the Baofengs and RTL-SDR dongles) that has a well designed preamp built in, below 70cm you will likely not notice any significant difference between RG-8X and LMR-600, especially if your station is carefully constructed to minimize common mode noise on the coax shield. The preamp on the front end of a decent receiver is quite powerful, and if all of the signals coming down the coax are attenuated uniformly (that’s why common mode isolation is important), it will just increase the gain and you’ll hear OK. This is why Beverage antennas, which can be 10dB or more below even a rather poor vertical antenna, are still exceptionally good receiving antennas.

That said, with a poor receiver, you may indeed notice a modest improvement in receive signal strength. The feedline pays for itself on the transmit side, the antenna pays for itself on the receive side.

On VHF/UHF, height is might. They higher the antenna, the more likely you are to clear obstructions in a manner where you are just simply dealing with free space path loss versus loss due to terrain shielding. The extra loss in a longer feeder to gain sight of the horizon may be worth it.

Remember, people will quote all those losses for RG-58 as if you gain that much by switching but it is the difference in losses between the RG-58 and the other coax that matters.

So, while it is good practice to eliminate as many losses as you can, you can at least try working with what you have.

At present the prices are about 60p a metre for RG58, 80p a metre for Mini8, £1.65 a metre for RG213 and £14 a metre for LMR-600