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Author Topic: How much better is an N over SO-239 at VHF/UHF?  (Read 9100 times)
NZ5N
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« on: July 13, 2007, 10:47:21 AM »

As almost all upper-end 2 meter and 70 cm beams come with N connectors, I was wondering how much of an improvement they offer over an SO-239.  I picked up a 2 meter beam which has no connector at all, the coax is supposed to be attached directly to the antenna, but it would be a lot more convenient if I could add an SO-239 (or alternatively a short length of coax with a PL-259 and a barrel connector, but I suppose this would be even more lossy).

I'll go with the N if it's worth it, but I recall a recent thread where it was shown that, contrary to popular belief, barrel and elbow connectors have almost no loss, at least on HF.  So what are the facts regarding connectors at VHF/UHF?    
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NZ5N
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« Reply #1 on: July 13, 2007, 10:52:19 AM »

Well, at least one ham thinks that UHF connectors are as good or better than N connectors at 70cm:

UHF Connector Maligning -- There are many misinformed engineers and amateurs who have been led to believe that a UHF connector is the worst thing ever invented in the RF world -- due to it's lower internal impedance. They believe that each UHF connector causes a 1/2 dB insertion loss and a whole lot of VSWR at 432 MHz. I've heard quite a few amateurs claim that their 432 MHz brick amplifier will now have 1 dB greater gain since they just replaced the two chassis mounted UHF connectors with Type N connectors. This "Old Wive's Tale" has been propagated for decades. Everyone believes it. No one challenges it. Few people have ever make the measurement.

A High Power "Calorimetry" Test -- Here is my observation. I took a 432 MHz Stripline Parallel Kilowatt Amplifier and applied 700 watts through a UHF female and a UHF male connector, and then into my antenna feed line. After 10 minutes of 700 watts throughput power the UHF connectors were mildly warm. If I estimate that "mildly warm" represents a dissipation of 3 watts out of 700 watts, that's an estimated insertion loss of 0.019 dB for the pair of connectors. You're about to ask, "how can this be, the internal dimensions are approximately a 35 ohm impedance, it's got to cause a 1.43:1 VSWR?" Well, it doesn't.

Very Little Total System VSWR -- The mated UHF connector has an internal connector length of less than 0.9 inches. A free space wavelength at 432 MHz is 27.3 inches. The 0.9 inches represents a phase length of 11.9 degrees. If I plot this up on a Smith Chart (or use the mathematical equivalent) I find the following. A 50 ohm antenna with an 11.9 degree long section of 35 ohm line causes an input impedance of (47.9 -j7) ohms. That's an input VSWR of 1.16:1, which gives a worse case reflected-power-caused transmission loss of 0.024 dB. To me that's insignificant. Now, I'll admit that at 10 GHz, where the wavelength is 1.1 inches, that 0.9 inch electrical length connector would be much harder to tolerate.

Power Tolerance -- A Type N connector can tolerate low-duty pulses of over 20 kilowatts without a voltage break down. However, steady state power of more than 1 kW could cause the connector to fail from the RF current overheating the center pin. Most connectors have a very similar failure mechanism when steady state high RF power is applied. The UHF connector has an oversized center pin that can more easily tolerate high steady state RF currents. Moreno said that 30 ohms impedance maximizes the power handling, and the UHF connector has an impedance of about 35 ohms.

Each EME'er who is using those expensive type SC connectors on his kW amplifier could probably use UHF connectors for his indoor cable attachments, if he desired to save money. The UHF connector has a larger center pin than an SC connector, it might actually have a larger power tolerance than the SC -- this will require testing. But, remember that the Fluoroloy-H dielectric on the SC connector is designed to be a good heat sync that cools the center pin.

It's User Friendly Assembly -- There are probably twice as many amateurs who can do a good job of installing a UHF connector on an RF cable, as compared to a Type N connector. The proper installation and WX proofing of a Type N connector requires considerable finesse and experience. It's almost an art form.

UHF Connector Faults -- There are two major faults I can find with a UHF connector when it is being used on 432 and below: (1) the lack of weather proofing; (2) the lack of outer conductor finger contactors. With a proper tape wrapping job, I believe the weather proofing can be accommodated. However, the user must be sure that the internal "teeth" are properly seated, and that the outer nut is kept tight; otherwise the outer conductor can develop a considerable growth in electrical length, with the associated "scratch contacting" noise. For this reason the connector is probably inappropriate for a high vibration environment, unless an auxiliary nut-retaining mechanism is employed.

So, maybe it's time we stop saying such bad things about the poor-orphaned UHF connector. For our purposes, it doesn't deserve all that flack. Properly used by a savvy engineer, who understands the idiosyncracies, it can give you a lot of bang for the dollar. It's been around for 60 years, that's no coincidence.

I welcome alternate opinions on all of the above. Please feel free to correct the mistakes.

73 es Good VHF/UHF/SHF DX,
Dick, k2RIW.
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WA3SKN
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« Reply #2 on: July 13, 2007, 11:20:05 AM »

The PL259/SO239 connectors will show an impedance "bump" around 300 MHZ, and is not weather-proof.  It is in-expensive, and almost universal now til you get to UHF frequencies.
The N connector is called a "constant impedance" connector. It IS weatherproof, and a more difficult installation... Plus... more expensive.
Depending on how much of a "perfectionist" you are will determine your preference!

-Mike.
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WA3SKN
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« Reply #3 on: July 13, 2007, 11:26:25 AM »

Oh, on the 2 mtr antenna... I would go with an 1/2 or 1 wavelength piece of coax and a PL-259.  Of course, this would depend on how you plan on matching it!

-Mike.
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N3JBH
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« Reply #4 on: July 13, 2007, 11:54:15 AM »

I have to ask this question. What is so hard or bad about attaching the coax directly too the yagi? I have  a few hy-gain vhf antenna’s here all use the direct attachment of coax to the driven element. I love it.

Now that being stated your asking about losses of the 2 connectors. If the added loss of either connector is enough to cause you concern. Why not simply avoid them in the first place like the designer  did?

Please don’t take this as I am trying to be a smart a$$ here. I really am not. I am  just asking why  even consider it in the first place. Because your going to have to make the connections using coax to the yagi anyways and by adding any connector is only going to add to loss.

So now that I said all that I will admit I am sorry if I sounded off topic in any fashion. I just wanted to offer some food for thought was all. Jeff n3jbh
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WB2WIK
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« Reply #5 on: July 13, 2007, 11:58:34 AM »

Type N's aren't really weatherproof, either.  They *could* be, if the cable and rear nut diameters exactly match, which they almost never do unless exactly the right connector is used for exactly the cable it's being mated with.  (A rarity in amateur radio circles.)  When water gets in past the jacket seal at the rear nut, it can get around the internal gasket easily.

But anyway, I have never seen any difference at all between properly installed UHF connectors and properly installed N connectors below about 400 MHz.  At 432 MHz, there's a very small difference, favoring the type N.  At 1 GHz, there's a much bigger difference, favoring the type N.

As Dick K2RIW pointed out in the "copied" post above, the UHF connector male can handle a great deal more power than an N connector can.  With a UHF mating pair, the weak link is the female connector: The male is almost perfect.  The N male and female, on the other hand, are virtually identical except for the center pin or contact, both of which are undersized compared to a UHF mating pair.

If you have a 2m beam that has no coaxial connector as part of the feedline attachment design, be careful to do this job right.  It's easy to "wire" a piece of coax to the driven element of a 2m beam using "pigtails" from the coax that are much too long and result in a 2m beam that resonates down around 135 MHz.  Happens all the time.

WB2WIK/6
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NZ5N
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« Reply #6 on: July 13, 2007, 12:15:24 PM »

Thanks for the replies, interesting stuff.

"What is so hard or bad about attaching the coax directly too the yagi?"

Nothing really, as long as kept the pigtails are short (as Steve notes) and proper strain relief is provided.  At my shack, unfortunately, several antennas have to share a single feedline, so until I get a switch I have to move the coax around sometimes, and this is easier with a PL-259 on the end.  Also makes it a little easier for portable use.
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K0BG
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« Reply #7 on: July 13, 2007, 12:32:55 PM »

Part of the issue here is "...properly installed..." means different things to different people. While just about every amateur will sooner or later install a PL259, the vast majority ARE NOT installed properly.

Part of the problem is this; the trim lengths mentioned in most popular publications, including the the ARRL Handbook, are incorrect! There is even one on-line source which suggests cutting foam core coax differently than solid core. What's with that?

The three common mistakes are: The shield shows, the barrel solder holes are not filled completely, and the tip is not filled with solder.

Lastly, very few amateurs use heat shrink over the barrel end, much less double walled, glue impregnated heat shrink. Installed properly, soldered properly, and heat shrunk properly, a PL259 will be weatherproof for years.

Alan, KØBG
www.k0bg.com
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N6AJR
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« Reply #8 on: July 13, 2007, 12:39:29 PM »

Hey Little Brother,

 The Biggest problem I have in conecting a coax directly to the antenna ( especially Yagi's) is the fact that sooner or later you need to change the coax or the rotor or something or move the antenna.

And it is so much easier to swap a coax, rather than to redo a straight coax to antenna joint. Also you need to loop the straight coax up in a half loop and down so the place wehere the 2 pigtails come out is pointind down to keep water out of the coax.  

Mostly its just easier to hang a connector on it, but you can run straight in on most circumstances..
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N3JBH
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« Reply #9 on: July 13, 2007, 12:47:17 PM »

Hey Big brother
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NZ5N
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« Reply #10 on: July 13, 2007, 01:05:58 PM »

How bad are barrel connectors at VHF/UHF?
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K6AER
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« Reply #11 on: July 13, 2007, 02:18:46 PM »

I have measured UHF and “N” connectors at 50, 146, 450 and 900 MHz on my network analyzer. Below 150 MHz there is no difference in insertion loss. At 450 MHz the UHF connector had about .1dB more loss. At 900 MHz the UHF connector had .23 dB more loss than the “N” connector. These measurements were made with an HP 8753 network analyzer.

Above 900 Mhz the uhf connector start to degrade quickly. Contrary to belief there is no impedance bump below 450 MHz. The main draw back to the UHF connector is the inside contact area is not weather proof and can tarnish over time. Also the mechanical connection is not as good as a “N” connector.

I find crimp on “N” connector much easier to put on coax and prefer them to the solder on types.
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WB2WIK
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« Reply #12 on: July 13, 2007, 02:50:11 PM »

>RE: How much better is an N over SO-239 at VHF/UHF  Reply  
by NZ5N on July 13, 2007  Mail this to a friend!  
How bad are barrel connectors at VHF/UHF?<

::They're actually very good, if you use good ones.  A real, mil-spec type PL-258 (double female adapter, UHF) from Amphenol has immeasurably low loss through at least 400 MHz.

The type N barrels are very useful (I use them all over the place) up to 6 GHz, and really good ones to 12 GHz.

WB2WIK/6  
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K0IZ
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« Reply #13 on: July 13, 2007, 03:23:57 PM »

Radio Works sells an N male connector that is as easy to install as a PL259.  Costs about $3.50.  
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W8JI
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« Reply #14 on: July 13, 2007, 05:20:36 PM »

It's all about the length and amount of the impedance bump.

The real truth about this is almost ALL of the loss and impedance bump is in the female. The male is nearly perfect.

The main area of the female that has the impedance bump is the flared area where the pins mate. That area, with a typical dielectric, is about 35 ohms and about 1/2 inch long. that really is NOT a major bump.

Now if you use a hood over the open end of the female and keep the leads short, the UHF female connector has no issue at all up to perhaps a few hundred MHz.

Below about one hundred MHz the UHF, if good quality, has higher power handling than the type N.

As far as loss, it is extremely low in most UHF connectors. You cannot normally use a network analyzer to measure the real loss, since the network analyzer considers mismatch part of the loss. In our gear, since it is generally not a 50 ohm pure resistive source, mismatch loss doesn't apply. It certainly does not apply to heating!!

When I measure connector loss I use temperature rise, since that indicates the dissipative loss. The result is you have a transmission bump that affects SWR and a dissipative loss that represents heat. The network analyzer combines the two, giving an absolute worse possible case loss that counts all impedance mismatch as dissipation, when it almost never is in our applications.

Female barrels can be problematic if they are long and if the manufacturer wasn't careful. They key is to use the shortest barrel and a good quality barrel if at high power or at high frequencies.

At 350 MHz or higher I always strongly consider N connectors, but I never use them at lower frequencies unless it is an SWR critical application like measuring impedances.

As a matter of fact when I get used Heliax with N connectors, the first thing I do is machine the N connectors into UHF style connectors!! The UHF style is significantly more reliable with lightning or high power. Now for UHF I stick with N's, but not anything VHF or lower.

73 Tom  





 
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