Other SOTA sites: SOTAwatch | SOTA Home | Database | Video | Photos | Shop | Mapping | FAQs | Facebook | Contact SOTA

MTR3b - RCA socket vs. BNC pros/cons? Modify?


#1

Hi, while almost there with my Holy Grail - while playing my MTR3b I found out today that the RCA/BNC failed connecting. I only found this out because of having a power/swr meter connected between the trx and the antenna. Lucky me.

Anyway, I was wondering - is there any advantage of RCA over BNC? I find this a bit troublesome, as all my radios have BNC plug and only MTR 3b has the RCA. When I wondered advantages of RCA - the only one coming to my mind is that if my feeder plug fails, I can always stick the wire in the RCA socket, tighten it with a wood stick and still be in the game. Not sure if this was the concept of LNR to equip the radio with the RCA.
Do you see any pros in favour of keeping the RCA in my MTR3b? If I recall well I have seen this modification done by someone already, but maybe I am wrong.

Happy to hear your thoughts on the pros/cons of the RCA and possible modification to install BNC.


#2

Hi Konrad,

Presuming the particular RCA connector you are referring to is what is also called a phono socket? There’s a reason they’re called Phono sockets - they’re meant for Audio, not RF. Although at low power they will work fine on HF, I would certainly replace the socket with a BNC socket.

I presume from what you are saying you currently use an RCA/Phono to BNC adapter in any case, so you’d be getting rid of one more possible failure point in the sequence if you switch to a case mounted BNC socket.

Make sure you get a 50 Ohm BNC socket though (there are also 75 ohm versions where the centre conductor is a different size).

73 Ed.


#3

Hi, Ed - yes, removing extra point-of-potential-failure and extra carry-on is one of the reasons… :slight_smile:

However still - I am wondering why this one was equipped with the RCA type of antenna socket.


#4

Hi
The RCA is smaller and cheaper.

73
Ron
VK3AFW


#5

I swapped my RCA out for an SMA, partly because it means more of my patch leads will work with both my HF and VHF gear, and because my analyser requires an SMA to MCX adapter. I agree with the thinking behind the RCA: they are certainly easier to connect in winter with thick gloves on, whereas SMAs can be a hassle at the best of times. Is it the RCA socket on your MTR that is defective, or the plug on your coax? Perhaps you could try a better quality RCA plug before swapping out the connector on your MTR, they have their advantages.
de OE6FEG
Matt


#6

Thanks, Matt. The TRX is fine including the socket, it is just the adapter losing connectivity. I would be happy to buy a high quality one, but I found only cheap ones = <1 EUR…


#7

Look for an RCA connector made by Neutrik. A local ham is a retired audio engineer and always recommends Neutrik.

http://www.neutrik.com/en/phono-rca/

Here is one for $3.

wunder


#8

The MTR is designed to be a rig that you take with you out into the wilds. You’re more likely to find a shop selling phono connectors than selling RF connectors.

BNC connectors are heavier and more expensive than phono connectors.

Despite all the ‘snobbery’, phono connectors work fine for QRP HF, there’s no good reason to change them on gear other than convenience of connecting to other gear. My two band MTR antennas are terminated in phono plugs, they plug straight into the rig. Most of my gear uses BNC, it’s my ‘standard’ connector for RF.

73, Colin


#9

RCA connectors were originally intended as a r.f connector. Back in the early 70’s I worked for a two way radio manufacturer and RCA connectors and cables were used for interconnecting various circuit board modules.

Later when when building qrp h.f ham radio sets of various sorts I used RCA connectors which worked quite well with no problems.

Now have pretty well moved over to BNC’s where ever possible so as to standardise all my interconnect cables.

73’s Wal VK2WP


#10

This is a very interesting question !

Primary advantages of BNC:

  1. Near-perfect 50 ohm impedance at HF and VHF
  2. Gold or silver plated center contacts in better BNC connectors
  3. Good reliability
  4. Economical and widely available
  5. Many varieties designed for PCB’s, panels, etc.
  6. Female is physically strong and forms a rigid termination to the mounting surface
  7. Plugs offer excellent RF shielding
  8. Many nice precisely engineered and machined parts that engineers appreciate

Disadvantages of BNC:

  1. Requires rigid mounting surface - not good on thin material
  2. Relatively heavy for low power - a male and female connector pair weigh more than 1 oz (~28G)
  3. Male connectors must be terminated with care and attention to details
  4. Male plug fails with repeated stress on cable - shield separates
  5. Difficult to repair or improvise fix on a summit
  6. Plug must be aligned with female to mate, then rotated to lock
  7. Plug is difficult to deal with in really cold weather, and touching the cold metal freezes the bare fingers
  8. 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
  9. Different coax cables require specific male plugs with correct dimensions
  10. Both male and female connectors are relatively bulky for tiny radios
  11. Female protrudes significantly from mounting surface and can be damaged
  12. Male connector cannot be rotated to correct cable twist
  13. In cold weather, coax cables become stiff, so the rotation of mating the plug stresses the shield more then usual

Advantages of RCA:

  1. Very low cost
  2. Widely available
  3. Very simple compression connector contacts
  4. Both contacts can be adjusted for correct compression
  5. Termination to cable is very easy
  6. Field fix is possible if cable breaks at the plug
  7. Impedance is close to 50 ohms, and length is very short, so mismatch is trivial at HF and VHF
  8. Mating does not require alignment, so mating is quick and easy
  9. Mating force is low
  10. Easy to mate with gloves in cold weather
  11. If touched with bare fingers, surface is small, and force is low
  12. If cable is jerked, plug comes loose, often with no damage to anything
  13. Very light weight - a few grams for a pair
  14. Almost no torque required to mate and unmate
  15. Female connector can be mounted on almost any surface, since torque and mating force are minimal
  16. Because of (15) above, enclosure of device can be lighter if desired
  17. Cleaning contacts is easy
  18. Handles RF power OK - probably good for 100W at HF
  19. Available in many styles for PCB, panel mount, etc.
  20. Females are very small, ideal for tiny QRP radios
  21. Females protrude from surface by only a few mm
  22. Females have a curved surface and are rugged
  23. Modern plugs offered with low-cost plastic hoods, as well as metal shielded versions
  24. Plugs with removable hoods are easy to inspect and service
  25. Plugs work with a variety of small coax cable types, not cable-specific
  26. Cables with male plugs are so light, it’s practical to take a spare
  27. Plug is easy to rotate to correct cable twist

Disadvantages of RCA:

  1. Plug may come loose if cable is pulled
  2. Impedance is not precise enough for instrument use, UHF, microwaves, etc.
  3. Contacts are often tin or nickel-plated, so they can develop resistance sufficient to cause temporary failure, especially with the cheaper types
  4. Most male plugs have little strain relief, so they may fail if subjected to excessive or repeated force
  5. Often used for audio, but cheaper types are not ideal for low-level signals
  6. Plastic-hooded plugs offer little shielding
  7. Not designed for high-power RF
  8. 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


#11

Hi George
Some RCA plugs used to have a larger diameter pin which spread the female socket and caused grief when a “standard” RCA plug was later used.

Don’t lnow if the two sizes are still sold.

73
Ron
VK3AFW


#12

I can see no issues with an RCA / Phono socket and plug for QRP HF. Even the tattiest of the cheap connectors will work. So swapping BNC for Phono or Phono for BNC is fine.

Despite trying to minimise the types, I still have PL259, BNC and SMA connectors on my SOTA gear. On BNC males, I always file down the over-centre part of the collar so it doesn’t need much effort to lock in place. This is simply to save wear on the lugs on the sockets on the radio etc.


#13

Agreed Andy, I have been using phono plugs/sockets for a long time on HF, with upto ten watts and no problems. I could never master fitting the bnc plugs properly so was glad the phono’s work.
I use cheap plastic plugs and also have them connected to my vx6 antenna on 2 metres.
Mike


#14

OK, thank you all. So it seems I need to give it a try with the RCA - perhaps I will make some dongles to adapt to BNC, as I have all my cables already done for BNC, they are heavy duty, some military spec or even military-origin, so cutting them and making RCA’s would break my heart… :slight_smile:

Thanks, again!


#15

Another advantage to RCA is that it is easier to shove a wire in or otherwise jury-rig if you forget an adapter.


#16

Greetings. It can an be noted that Collins Radio used RCA sockets designed for RF in the KWM-2 rig. 73 de Geoff vk3sq


#17

RL Drake used them too, for small signal interconnects.

Adrian
G4AZS


#18

Yes, my KWM-2 uses RCA for about every connector short of the power connector and mic connector. I regularly pump 100 watts out the almost 60 year old RCA antenna jack.


#19

MUCH prefer the RCA jack and plug to BNC in this service. IF using any coax at all, I usually don’t, connecting BNC plug to RG174 is difficult at best, unreliable at worst. I do NOT like the BNC on my MTR5, haven’t replaced it because the hole for the BNC is bigger than a hole for an RCA.

As pointed out, I don’t use coax at all for SOTA operations. I have built a tuner for EFHW so small that it fits into a dental floss case and weighs 18 grams. This is plugged directly into the MTR3 RCA jack. The end of the wire plugs directly into the tuner with a 2mm plug.

two versions of the tinsy tuner for an EFHW which plugs directly into the MTR3 (or MTR5 with a BNC adapter in between). Antenna wire plugs directly into it, no feedline. There is (essentially) no RF current on the end of a halfwave wire.

Keep things simple, small, and light.

73, Fred KT5X (aka WS0TA)


#20

Hi Konrad,

for exactly the same reason I replaced the RCA socket in my MTR3B with BNC before assembling the kit. It was almost no effort, just increase the diameter of the hole a bit

All advantages of RCA may be true but didn’t convince me to cut my cables. And I didn’t want to use adapters which can easily get lost or develop conact issues.

73, Roman - DL3TU