N and UHF connectors have their pros and cons. The same is true of other connectors, be they BNC, SMA, F, DIN and the many other RF connector series. I use them all, by choice or by necessity when present on equipment I use.
There is no one correct connector. Each is designed to meet different engineering requirements. Connectors themselves come in many varieties -- chassis mount, PCB mount, screw on, solder, crimp, clamp, mechanical -- to suit different environments and applications.
The majority of hams use UHF connectors for RF connections. They are ubiquitous on HF transceivers, except those with very small enclosures. They perform well, are inexpensive and easy to use.
V/UHF operators tend to use N connectors to avoid the impedance "bump" of UHF connectors that can be problematic at higher frequencies. UHF connectors do not preserve the Z₀ of the coax throughout their lengths, whereas N connectors do. There are different N connectors for 50 Ω and 75 Ω systems. There is no such differentiation for UHF connectors.
The reason for the impedance discontinuity in UHF connectors is that there is a short air gap between the end of the coax and from the mechanical structure of the termination, in particular the insulator surrounding the centre pin. The dielectric constant and conductor separation vary over that span. We have effectively inserted a short transmission line with a different nominal impedance. Mechanical design takes priority over impedance performance in UHF connectors. The impedance bump can be worse for coaxes smaller and larger than RG213.
A typically quoted value for the bump is 30 to 35 Ω, which is believable from inspection of the transmission line equation for coaxial cable. Let's use the lower value even though there are differences across manufacturers and adaptors for large and small coax diameters. I'll further assume that the length of the discontinuity is 1 cm (10 mm or 0.4"), with a VF (velocity factor) of 0.7. Choosing these values should simulate a worst case for PL259 (male) UHF connectors.
I used SimSmith to insert a 1 cm long section of 30 Ω coaxial transmission line between a 50 Ω generator and a 50 Ω load. You can see that the effect is negligible at 50 MHz. It is marginally significant at 144 MHz and notable at 450 MHz. Of course there will be more than just one connector in a transmission line, with one at the rig, one at the antenna, two at each coaxial joint (e.g. barrel connectors) and two at each intermediate device such as antenna switches. The one at the antenna is usually of no interest since the feed point matching system typically compensates for that connection. The same is true of the transmitter or tuner at the generator side of the line.
SWR does not sum arithmetically. You can't simply add up the fractional quantities to discover the net SWR (mismatch) due to multiple connectors. That is, two connectors that exhibit a 1.05 SWR in a 50 Ω system when properly terminated do not give an SWR of 1.1. In select cases, insertion of a short 30 Ω transmission line can improve the match.
In a real system there are many variables that a simple analysis like mine cannot possibly contain. In each system you'd have to measure the complex impedances and coax lengths for each band of interest, along with the major effects of the load mismatch (no antenna is perfect!), generator action and other factors. My SimSmith model is illustrative but not universally applicable.
My usual recommendation to those who ask is to avoid N connectors from 160 to 2 meters. The performance impact of using UHF connectors ranges from negligible to modest, and is usually dominated by the antenna impedance, which is usually nowhere near 50 + j0 Ω. Other factors such as variation in coax impedance by manufacturer, application and age have impacts greater than that of the bump due to UHF connectors
None of this is a criticism of N connectors in general. It is reasonable to ask: why not use N connectors? Why not sweep away the uncertainties by avoiding UHF connectors wherever feasible? There are good reasons to prefer UHF over N connectors other than that of the dreaded impedance bump.
Pieces
The typical N connector has several components. It is an unwise ham who breaks open the package without a plan. It is easy to lose one or more of them, especially the centre pin.
I opened one of my bags of used N connector parts so you can have a look. These are 50 Ω connectors that were salvaged from old coax. Most of the pieces are present, and I have other bags with different assortments. I have fewer pins than bodies, which is annoying.
The main gasket is cut in two when first assembled, but they can be reused if you are very careful. Alternatively, do an especially good job weatherproofing the assembled connector. The gaskets in a pristine N connector provide good weather protection, but don't depend on it. Protect them as you would a UHF connector for reliable long life. Braided coax wicks moisture and will soon degrade.
Used N connectors can be frustrating unless you have a good eye for the slight size difference between 50 Ω and 75 Ω pins. I am not so worried about the impedance than I am about trying to mate mismatched pins. It doesn't work and you can't force it. Force fitting a male 50 Ω pin into a 75 Ω female pin will destroy it. UHF connectors don't have that risk.
Notice that I've shown soldered connectors. That is only for the centre pin since the braid is mechanically bonded when the external nut is tightened. However, you must be precise with the dimensions of the exposed braid, dielectric and centre conductor or the braid will be loose. It's a problem I've encountered many times with connectors I've assembled and with those that have followed me home.
A related concern is the position of the centre pin. Get the dimension wrong by just 2 mm and the male and female pins either won't make solid electrical contact or will bottom out and bend when the connectors are joined. Again, this can't happen with UHF connectors.
There is another challenge with pin alignment when using RG213 and similar polyethylene dielectric coaxes. The braid can slide over the dielectric due if too much of the coax weight is taken by the connector. The same will happen by weather induced thermal expansion. It takes little movement to have a connector suddenly fail in the middle of a winter contest due to lost electrical contact or arcing. It is more common than you might imagine.
I had to deal with improperly aligned N connectors on a fellow ham's tower last year. If you have trouble properly preparing the coax and assembling an N connector on the ground, well, it's far worse doing it up in the air. The cables with the poorly aligned connectors were cut down for repair on the ground and then lifted back onto the tower and tied down to ensure there was no weight (stress) on the connectors. One of them failed again a few days later. This time we decided to cut it off and friction fit a UHF connector to the coax. Half a year later it's still working despite not being soldered.
Contact Area and fragility
Alignment is especially problematic with solder connectors on flexible cables with a stranded centre conductor (e.g. RG213). Getting the pin centred must be done manually. A small deviation off centre can easily damage the pins when joining connectors. The hole in the female pin is very small and easy to miss since once your view of the pins is hidden when the connectors are brought together. You must be careful and go by "feel" alone.
Just 1 mm of linear insertion error due to the pins projecting too little or too much results in inadequate contact or bending, depending on the direction of the error. In the former case, the connection is unreliable either due to high resistance or arcing with high power. In the latter case the constant impedance of the N connector is lost and there can be a short or arcing when the pins are bent. The outer spring flanges of the male connector are usually not a problem because they are stronger and their position fixed by being bonded to the connector body.
Threading ensures proper positioning of the male flanges against the body of the female connector. Since the threads are so fine -- ⅝"-32 UNEF -- crossing threads is easy, which can damage the threads and the connector flanges. Damage is easier with hard line coax since it is more difficult to manipulate the cables to ensure they're properly aligned when the threads are engaged. On a UHF connector, the large centre pin engages first and guarantees proper alignment of the contacts and thread engagement.
Above is a splice between a male N connector on buried LDF4 cable to my 80 meter vertical wire yagi and a female N on the long LDF5 run to the station. This week I discovered that the flanges are making intermittent contact, and that is the reason for the antenna's failure this winter. My guess is that I bent something slightly when struggling to align the connectors for thread engagement. I have it working again but I may replace one or both connectors this summer just to be certain that it survives next winter.
The centre pin and female spring flanges on a UHF connector are large and robust in comparison to those of N connectors. They are not easy to damage, yet there are hams who manage to do it anyway! If the flanges are overspread they can often be fixed, at least in an emergency, using a small flat blade screwdriver to bend them inward. The large contact area of the centre pin and threaded shell assure good electrical contact even in cases of minor damage.
There are no gaskets in a UHF connector so it is mandatory to use external weatherproofing. Moisture infiltration will cause corrosion, and that leads to high resistance and arcing despite the large contact area. Never leave a UHF connector unprotected outdoors, even temporarily. Weatherproofing it after rain, snow or morning dew (condensation) will trap the moisture inside.
Alternatives
All is not doom and gloom! There are N connector styles that largely eliminate the problems of exact dimensions and pin alignment. These are so good that there is no reason to succumb to the temptation to buy or reuse solder N connectors. But it can be expensive if you have a lot of connectors in your station, as I do.
I am pretty well forced to use N connectors for the thousands of feet of Heliax in my station. They can be found surplus in quantity and at very attractive prices. If you know someone in the commercial wireless business you can often get them for free by dumpster diving. The Andrew connectors are very robust and I've had better than a 90% success rate reusing scrounged connectors.
UHF Heliax connectors are rare. Newer commercial installation often use 7/16 DIN connectors, and they are not yet showing up in quantity on the used market. The DIN connectors are larger and hardier constant impedance connectors. The ring of thick flanges on the female 7/16 DIN connector in the picture at right are for the centre pin, not the outer conductor!
I have several new and used DIN connectors in my stock. They are useful for splicing sections of Heliax but not at the ends of the feed line where you'll likely need an adaptor. I have just one N-to-DIN adapter in my stock and it wasn't cheap. I have never shopped for a 7/16 DIN connector for LMR400 to connect to Heliax DIN connectors. If they exist they are certain to be expensive.
These are my most recent flea market finds. A paid a modest price for these used LDF5 Heliax connectors for the convenience of just picking them up and walking away. Dumpster diving is free but there is effort involved. I've already started giving them away to friends. I have enough on hand and more will undoubtedly appear in the coming months and years. Surplus and "reel ends" Heliax is available at good prices if you are fortunate enough to have contacts with commercial tower service companies.
Adaptors for joining N and UHF connectors are common and inexpensive. The kind shown in the picture are often found surplus for a dollar. I also buy the less common female-to-female adaptors for when the Heliax connector is an N male.
I use many of these adaptors on the tower and on the ground to interconnect RG213 and LMR400 running to antennas and antenna switches. One good feature of adaptors is that the N side is always perfectly positioned. Alignment is never a concern.
Fitting the coax can be frustrating if you don't closely follow the installation instructions. The centre conductor must be chamfered with a file so that it can fit inside to lift the flanges as it is pressed in. Without the chamfer the conductor won't fit and no amount of pressing will help. The coax must be very straight for this operation. Any deviation must be corrected since there is little wiggle room to align it with the flanges once it's inserted, and you must do it blind.
Captivated connectors are not cheap. I purchased several at a good price when I was first building this station. I thought it would be easier and cheaper than using UHF connectors plus an adapter to the Heliax N connectors. I was wrong on both counts so I stopped using them. However they are excellent connectors and they are in wide commercial use for their reliability.
I couldn't find a good picture of an LMR400 captivated connector online so I took one of a single piece Heliax connector in my stock that uses a captivated centre conductor. The difference is that the Heliax centre conductor is hollow so the flanges grip the inside rather than outside of the conductor.
There are restrictions on the application of captivated connectors. They fit one and only one type of coax. You must buy the correct connector for the coax. Captivated connectors don't work on coax with a stranded centre conductor like RG213.
Two-piece Heliax N connectors are easier to install correctly than those for LMR400 or RG213. One reason I like using LDF5 in my station is because the cable and its connectors are widely available on the surplus market and the connectors are easier to install than on smaller Heliax (LDF4) and larger Heliax (LDF6 and LDF7). All it takes is a hacksaw, knife, file and wrenches to make a perfect termination.
More alternatives
Does all this information make your head spin? Do you really hate fooling around with coax connectors, but you still would like N connectors? There are many companies that will fit any length of coax with the connectors of your choice as a complete custom assembly. The prices I've seen are reasonable, though more expensive than doing it yourself. It is also no guarantee against future problems. The choice is yours.
With regard to UHF connectors, you can also use commercially prepared cables. Many fear damaging the coax through excess soldering heat and cold solder connections. If you do it yourself, I recommend a silver plated connector which takes solder with less fuss and therefore a lower risk of damage. Some like K3LR solder the braid to the outside of the connector body. But if you do it wrong the shell won't slide into place. The impedance bump is longer with this method so it is best to avoid it on VHF systems.
There are many inexpensive UHF connectors on the market that do not meet spec and are difficult or impossible to use. Buy from a supplier with a reputation for quality. If soldering is too much to handle, use crimp UHF connectors. Cost for the crimp tool is worth it when you have many connectors to prepare. You may be able to borrow a tool from a friend if you only have a few connectors.
I stick with conventional silver plated connectors from reputable dealers, soldered through the holes, and I rarely go wrong.
Some hams convert Heliax N connectors to UHF. You can do this because the exterior threads are the same for both connector series: ⅝"-32 UNEF. See the proof in the above demonstration. I prefer the risk of future mechanical woes of Heliax N connectors over the work to do the conversion. Consider it food for thought.
Mythology
In addition to the myth than UHF connectors do horrid things to your SWR at VHF, and even HF, there are others. Two examples are that N connectors can't handle a kilowatt or a high SWR. Both are untrue, or at least not all that different from the performance of UHF connectors.
If you ask a ham about coax connectors you will almost always get an answer, and it will be delivered with supreme confidence. Sometimes the answer will be correct. Dig deeper by asking why and what their experience is with those connectors. You should be able to quickly spot the pretenders. Be especially wary of listening to those who tell you what you want to hear.
Uncertainty leads to extreme behaviour: aiming for perfection or taking an anything goes approach. The first can be a poor investment of time and money. The second is asking for trouble. Make decisions based on solid knowledge, not mythology.
Wrap up
My guiding philosophy can be summed up pretty simply: use UHF connectors when I can, and use N connectors when I must.
Unless you have a particularly good reason to use N connectors you are better off sticking with UHF connectors for HF and VHF. My reason for using N connectors is all the Heliax in my station. If not for that there would little need to deal with the many challenges of N connectors.
Most hams don't need N connectors.
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