Replacing a stock UHF with an N connector

[DrMag]

Ok; let me start right off the bat with a qualifier that this is a "what if" question, not something I am actually trying to do.

I've been wondering about why amateur radio continues to use the old UHF connector design (SO-239/PL-259) when it's known to not have great qualities at VHF/UHF. There's all kinds of arguments out there over whether or not it's actually a problem, but I did find one page that gives some actual data, finding the connector has an insertion loss of about 1 db at 430 MHz.

Now I have an essentially brand new (I've used it maybe 3 times) FT-7900R which puts out up to 40 W in the 70 cm band. (I am seriously considering selling this radio, by the way; if anyone is interested, let me know.) The radio, of course, came with an SO-239 (Yaesu calls it an "M" type) antenna connector with the diamond-shaped, two-hole panel mount version. I can easily find an N type connector with the same panel mount style, and it got me thinking:

Since the radio can put out 50 W on the 2 m band, I suspect that the power limit on 70 cm is due primarily to the output connector, since -1.0 db from 50 W is 40 W. If I were to replace that connector with an N connector, would I see the 70 cm output increase to the full 50 W? What consequences might that have on the rest of the radio?

Does this indicate that a radio like the TM-V71A which has a 50 W output on both bands uses separate amplifiers on the bands, so it puts 63 W at 70 cm to counter the insertion loss and get the full 50 W output?

[Rerouter]

They are both RF connectors, providing there designed impedance is the same, and the insertion loss is lower, it should improve your tx power level.

As for actual power output, you would need a power meter. or measure the voltage across a test load of the systems impedance.

[TheSteve]

I'm sure the lower output power on 70cm has pretty much nothing to do with the connector. It is much more likely based on the output preamp/amp having less gain as the frequency is increased.
I do wish all radios VHF and up used N connectors, well I'd be OK if even HF radios used them.

[German_EE]

The PL259/SO239 connector is better than you think, especially if you use quality parts with PTFE insulation and assemble them correctly. Also, that big center pin can carry a lot of current and is much better than the tiny pin on a BNC.

[voltz]

Interesting tests carried out here:
http://www.iz2uuf.net/wp/index.php/2016/01/08/pl-259-vs-n-on-430-mhz/

No real difference in terms of insertion loss at all. But there may be other reasons to use N Types like mechanical sturdiness and waterproofing.

Should be easy enough to test yourself. Just use a watt meter with your 40 watt radio into a dummy load, then insert a pl259 adapter in the chain, do you see a 10 watt drop?. Doubt it.

[CJay]

The biggest problem with PL-259s is that they're almost always fitted incorrectly and/or are piss poor quality, they're not ideal at 70cm but they are good enough as long as you buy decent quality ones and fit them properly.

[denverpilot]

Not worth it until you get to extremely high power or extremely weak signals, neither of which an FT-7900 is going to need to do.

It's FM. Your most likely use case scenario is a repeater, followed by simplex, and any upgrades in the antenna system will serve far more bang for the buck in dB gain, than mucking around with the connector on the rig.

1 dB is very unlikely to matter in the typical use-case for an FM rig.

If you intended the question simply as a thought experiment, apologies. But for practical use, it simply doesn't matter.

[metrologist]

I always thought this class of gear should have used BNC connectors for convenience.

[TheSteve]

I always thought this class of gear should have used BNC connectors for convenience.

BNC connectors perform quite poorly with higher power levels(especially as they age) as they aren't mechanically secure.

[David Hess]

I always thought this class of gear should have used BNC connectors for convenience.

BNC connectors perform quite poorly with higher power levels(especially as they age) as they aren't mechanically secure.

I agree.  BNCs are convenient in mechanically benign environments where ease of use is desired but threaded connectors like UHF, N, and TNC are much more mechanically robust.  I wish TNCs were used more especially on handheld radios.

[alm]

PTFE-insulated UHF connectors were also used for high-impedance connections before triaxial connectors were common. Early Keithley electrometers (impedance in the TOhms) and electrometer cables used UHF connectors. They were later replaced by TRB connectors (first two-lug, then three-lug). I have an adapter from two-lug TRB to UHF somewhere.

[DrMag]

If you intended the question simply as a thought experiment, apologies. But for practical use, it simply doesn't matter.

Yes, this is a thought experiment. It comes about because all the major manufacturers seem to make two versions: one for US/Japan with the UHF, and one for the rest of the world with the N. If there's little impact of a UHF on the signal, why make two different versions?

There could be other factors driving the R/E divide (using the Yaesu nomenclature) that aren't immediately apparent, of course. It's just something I've been pondering for the past while.

Thanks for all the comments! I'd love to see more and try to discuss and understand why the different connector choices are made.

[ahbushnell]

The PL259/SO239 connector is better than you think, especially if you use quality parts with PTFE insulation and assemble them correctly. Also, that big center pin can carry a lot of current and is much better than the tiny pin on a BNC.

Type N is not a BNC. 

[WastelandTek]

I have done some measurements on the PL259/SO239 and they weren't that bad.  They are certainly dated, but pretty fool proof and robust.

My understanding is that they are a holdover from WW2, and I can easily imagine a signal corps GI digging a jumper out of the mud, smacking it a few times on his boot or jeep tire and it just working.  With an N connector you better have Q-tips handy in that situation.  I have replaced SO239 with N female in some gear, but I certainly would not expect +10W out of your setup.

[bjcuizon]

How 'bout the good ol' binding posts and ladder line. 

[bjcuizon]

Oh, but then you need a tuner for it...oops 

[vk6zgo]

The PL259/SO239 connector is better than you think, especially if you use quality parts with PTFE insulation and assemble them correctly. Also, that big center pin can carry a lot of current and is much better than the tiny pin on a BNC.

Type N is not a BNC.

It still has a small centre pin---- if you don't have a BNC connector, an "N" can be pushed onto it, work correctly, & not damage the socket.
Cautionary note:- This only works with 50 Ohm connectors---- 75 Ohm "N"s have a bigger centre pin, & will wreck the BNC.

[xmo]

vk6zgo wrote: "...if you don't have a BNC connector, an "N" can be pushed onto it, work correctly, & not damage the socket."
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Don't do this.

I used to offer that tip based on learning it from a mentor.  He was wrong.

Yes the 50 ohm N will push onto a BNC, but the N50 center pin is considerably larger than a BNC center pin. Although it's not as drastically larger than the BNC as it is compared to the N75, it is enough larger to degrade the BNC socket.

Center pin diameters:

N50: 0.0645"
BNC: 0.0535"
N75: 0.0365"

Various spec sheets will show slight variations and tolerances for the dimensions but clearly the N and BNC center pins are not the same diameter.