This is a very interesting question !
Primary advantages of BNC:
- Near-perfect 50 ohm impedance at HF and VHF
- Gold or silver plated center contacts in better BNC connectors
- Good reliability
- Economical and widely available
- Many varieties designed for PCB’s, panels, etc.
- Female is physically strong and forms a rigid termination to the mounting surface
- Plugs offer excellent RF shielding
- Many nice precisely engineered and machined parts that engineers appreciate
Disadvantages of BNC:
- Requires rigid mounting surface - not good on thin material
- Relatively heavy for low power - a male and female connector pair weigh more than 1 oz (~28G)
- Male connectors must be terminated with care and attention to details
- Male plug fails with repeated stress on cable - shield separates
- Difficult to repair or improvise fix on a summit
- Plug must be aligned with female to mate, then rotated to lock
- Plug is difficult to deal with in really cold weather, and touching the cold metal freezes the bare fingers
- Since plug is locked to female, excessive force on cable (human error, or result of high wind problem) may either damage the cable, or jerk the radio, perhaps damaging it
- Different coax cables require specific male plugs with correct dimensions
- Both male and female connectors are relatively bulky for tiny radios
- Female protrudes significantly from mounting surface and can be damaged
- Male connector cannot be rotated to correct cable twist
- In cold weather, coax cables become stiff, so the rotation of mating the plug stresses the shield more then usual
Advantages of RCA:
- Very low cost
- Widely available
- Very simple compression connector contacts
- Both contacts can be adjusted for correct compression
- Termination to cable is very easy
- Field fix is possible if cable breaks at the plug
- Impedance is close to 50 ohms, and length is very short, so mismatch is trivial at HF and VHF
- Mating does not require alignment, so mating is quick and easy
- Mating force is low
- Easy to mate with gloves in cold weather
- If touched with bare fingers, surface is small, and force is low
- If cable is jerked, plug comes loose, often with no damage to anything
- Very light weight - a few grams for a pair
- Almost no torque required to mate and unmate
- Female connector can be mounted on almost any surface, since torque and mating force are minimal
- Because of (15) above, enclosure of device can be lighter if desired
- Cleaning contacts is easy
- Handles RF power OK - probably good for 100W at HF
- Available in many styles for PCB, panel mount, etc.
- Females are very small, ideal for tiny QRP radios
- Females protrude from surface by only a few mm
- Females have a curved surface and are rugged
- Modern plugs offered with low-cost plastic hoods, as well as metal shielded versions
- Plugs with removable hoods are easy to inspect and service
- Plugs work with a variety of small coax cable types, not cable-specific
- Cables with male plugs are so light, it’s practical to take a spare
- Plug is easy to rotate to correct cable twist
Disadvantages of RCA:
- Plug may come loose if cable is pulled
- Impedance is not precise enough for instrument use, UHF, microwaves, etc.
- Contacts are often tin or nickel-plated, so they can develop resistance sufficient to cause temporary failure, especially with the cheaper types
- Most male plugs have little strain relief, so they may fail if subjected to excessive or repeated force
- Often used for audio, but cheaper types are not ideal for low-level signals
- Plastic-hooded plugs offer little shielding
- Not designed for high-power RF
- People think it’s too cheap, and they will give you grief, even if it works well!
Here’s the bottom line:
My Elecraft KX2 came with a BNC female connector installed. This makes the radio about 12mm longer. If you use a small soft case for the radio, and stick it in your pack, the barrel of the BNC is going to take some force occasionally.
I got my KX2 in the winter, and I really didn’t like the BNC’s in cold weather! I had become used to RCA’s from my various ATS-3 rigs. KD1JV wisely took advantage of the RCA’s small size in the Altoids tins used for those radios. I liked the way they worked in the field, and I knew they would be OK.
After some study and anxiety, I removed the end plate of my relatively new KX2. This requires un-mounting the PA mosfets from the end plate, which is their heat sink. I carefully moved the BNC jack and replaced it with an RCA female connector. Fortunately I had one with the correct diameter threaded fitting to fit the KX2’s panel hole! I didn’t have to alter the internal cable more than a mm or two, and with a eye to the RF aspects of the change, I was pretty sure I was going to be OK.
I tested the KX2 in various ways, power output on all bands, zero SWR indication with 50 ohm load, etc., and the mod changed nothing of significance.
The radio fits my case easily, and I don’t have to think about plugging in my output cable, especially when my hands are already freezing, or I have on gloves. I can rotate the RCA plug to make the short coax cable lie flat where I want it to be.
I’ve done hundreds of activations with my KX2 since I made this small but significant change. I’ve used contact cleaner once to correct an imperfect condition with the RCA’s - that’s the only negative issue.
This unauthorized mod is probably one of the best things I ever did to improve my SOTA activations!
At the same time I put the RCA jack in the KX2, I removed the SMA jacks from my homebrew tuners, and replaced them with RCA jacks.
The SMA’s are even worse than the BNC’s, no matter how pretty the little gold jewels look in the sun!
73
George
KX0R
