Coax Impedance -- Concerning the possible choices of the impedance of a coaxial transmission
line, a great reference is "Microwave Transmission Design Data", by Theodore Moreno, Dover
Publications, 1948. On pages 64 through 69 he discusses four criteria for choosing a
particular impedance. The four choices displayed in the graph on page 64 demonstrates
how non-critical (broad ranged) many of these impedances are. Most of the following
addresses air dielectric coaxial transmission lines. Here are some interesting "Moreno" facts:
1. The maximum continuous power handling occurs at an impedance of 30 ohms.
2. The maximum breakdown voltage occurs at an impedance of 60 ohms.
3. The minimum insertion loss occurs at 77 ohms.
4. The maximum shorted line, resonant impedance occurs at 133 ohms.
5. Conductor losses (in dB's) are proportional to the square root of frequency.
6. Dielectric loss (in dB) is linearly proportional to frequency. Hence, at higher frequencies
the dielectric losses become increasingly important.
Cable Graphs -- We have all seen graphs of the insertion loss of our favorite cables. They
are usually displayed on Log-Log paper with the horizontal axis being frequency, and the
vertical axis being insertion loss in dB per 100 feet (or 100 meters). The curious thing
is that the insertion loss graph appears as a sloping straight line, with some of the cables
displaying a slight upward hook at the highest recommended frequency. Here is the explanation.
On Log-Log paper an exponential function appears as a straight line where the slope is
proportional to the exponent value. A square root function has a exponent of 1/2. A linear
function has an exponent of 1. On most of the cables, only the conductor losses (exponent of
1/2) are significant throughout much of the recommended frequency range. Thus, most of that
range is displayed with a slope of 1/2. The hook at the end represents the upper frequency
range where the dielectric losses are beginning to kick in. Here the line is beginning to
slide into a slope of 1.5, due to the combined effects of the 1/2 slope (conductor losses),
plus the 1.0 slope (dielectric losses).
Estimating Trick -- Knowing these facts allows you to make some interesting mental
approximations. Let's assume you know that your favorite cable has an insertion loss
of 1.0 dB per 100 feet at 144 MHz. If your friend asks you what's the approximate loss
at 432, here is what you can do. Since you know that the cable is usable to at least 2
GHz, you assume that conductor losses dominate throughout most of the 144 to 432 frequency
region, and conductor loss is proportional to the square root of frequency. 432 MHz versus
144 MHz is a 3:1 frequency ratio. The square root of 3 is 1.73. Multiply the 144 MHz loss
(1.0 dB) by the 1.73 factor, and you come up with a predicted approximation of 1.73 dB per
100 feet at 432 MHz. Because there will be a slight contribution due to dielectric losses
at this end of the cable's operating range you could round your prediction up to 1.75 dB
per 100 feet. Try this procedure on the graphs of your favorite cables and you will be amazed
how close the approximation usually is.
Cut-Off Frequency -- As you go beyond the manufacturer's upper recommended frequency, the
cable is capable of acting like a round piece of wave guide (WG). The presence of the center
conductor adds a little capacitive loading that slightly lowers the WG cut-off frequency.
Moreno recommends using this approximate equation for predicting the cut-off wavelength:
In other words, the limiting wavelength is approximately equal to the circumference at the
arithmetic mean diameter.
Coaxial WG -- Now, don't let this limitation always scare you into submission. The cable isn't
going to explode if you use it above the recommended frequency, it just gets a little tricky
up there. The first wave guide (WG) mode to consider is the TE11 circular mode. That's the one
used by the 10 GHz guys who are using 3/4 inch water pipe as a poor man's wave guide -- it
turns out to be a very high quality [low loss] wave guide. In the TE11 WG mode the maximum
E-field lines flow from the 6 o'clock position to the 12 o'clock position in the pipe
(vertical polarization is assumed). If your coax cable doesn't have any significant bends
in it, and the inner conductor is centered, it won't launch any E-field (WG mode) at right
angles to the center conductor. Your next question is "what's a significant bend?" The
microwaver's are going to have to study this, but, my gut feel is that a bend radius of
greater than 1 foot is OK.
It is just a matter of time until some smart amateur intentionally launches both propagation
modes in a piece of coax in order to lower the over-all insertion loss. It will require some
careful tuning of the launching structures at each end of the cable to insure that the two
modes end up co-phase at the top of the tower. This is because the phase velocity of the WG
mode is faster than the coaxial mode. This technique can only be applied to a narrow band
situation, or a set of narrow band situations (like 5 GHz and 10 GHz).
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.
Grid: FN30HT84DC27.
APPLICATION NOTES:
1. UHF Connector VSWR at 432 MHz
A 15 db return loss from a UHF connector that's being used at 432 MHz is quite good in many
circumstances. That return loss (a 1.43:1 VSWR) only causes an insertion loss of 0.14 dB
(before correction, such as re-tuning the transmitter). On the transmitter side of an EME
system, you'll never know it's there.
But, if there was a 15 dB return loss caused by a connector that's in front of a well tuned
LNA, that is significant. It could make a considerable difference to the system's Noise
Figure, if the operator did not apply VSWR corrective action -- such as tuning the LNA for
best Noise Figure performance while it is connected to the real system.
However, I suspect that very few of the currently operation EME antenna systems have a return
loss of better than 15 dB -- particularly not during rain and snow. Therefore, that savvy EME
operater has had to apply corrective action to the total antenna system, if he wants full
performance of his LNA. If the UHF connector is part of that antenna system, it will get
lumped together within that corrective procedure. Thus, that connector 15 dB return loss could
be very tolerable to a well-informed operator.
2. More 50 Ohm Magic, UHF Connectors
Introduction -- In various responses to my 31 May 2001 treatment of UHF connectors, cogent
comments were made that I wish to address, and add to.
Connector Brands -- Since the UHF connector doesn't seem to be protected by a MIL
Specification, there is a wide variation in the quality and mechanical performance of the
connectors that are available on the world wide market. The buyer must be wary. I hope that
a savvy amateur will create a web site list that will inform us of the UHF connector brand
names, and sources, that are worthy of our hard-earned money. Lloyd, N5GDB, and Lloyd, NE8I
both strongly recommend the silver plated or gold plated versions, particularly with respect
to solderability and connection integrity.
Installation -- I probably was too hasty when I stated that twice as many amateurs/engineers
can properly install a UHF connector versus a type N connector. An experienced RF maven (one
who has a "feel" for the way RF flows) can almost always suggest an improvement in the
connector installation procedure -- so that the lowest VSWR, least loss, best mechanical
strength, best longevity, and best weather proofing are realized.
Most of my outdoor equipment uses type N connectors, with BNC's most used indoors, and SMA's
used within enclosures. For the few UHF connectors that I use, here is my favorite connector
installation method.
(1) After properly cutting back the braid and dielectric, I next tin the braid (and center
conductor) with as little solder as possible, that will still coat the strands. Since the
end of the cable is completely open to air at this point, the amount of melting of the
polyethylene dielectric is minimized.
(2) I slip the nut onto the cable and then screw on the connector body. The tinned braid
causes extra resistance, and a strong pair of pliers are definitely required.
(3) Assuming that I've chosen a connector brand that readily accepts solder, the process of
tack-soldering through the 4 holes requires very little heating time, when using a
large-enough, hot-enough, soldering iron. Thus very little further melting of the polyethylene
dielectric takes place, and the complete braid is essentially bonded to the connector body.
(4) Clean off as much solder from the tip of the iron as possible, and heat up the side of the
center pin, while applying solder down the front hole. Try to keep solder off the side of the
center pin. If need be, wipe off any excess while it is hot. Excess solder left on the outside
of connector center pin will interfere with the proper mating with the female connector.
A further benefit of the braid tinning process is that the strands of the braid don't become
scattered, spread, and folded back during the process of screwing on the connector body. Thus,
full braid strength, and electrical bonding is assured by this process.
I suspect that other experts have further improvements on this process, and I welcome their
comments.
Crimp Connectors -- For indoor, non-critical applications I believe that crimp connectors can
be very expedient and handy. However, the crimping process has a number of characteristics
that bother me:
(A) True UHF Frequency VSWR -- For many crimp connector designs the outer braid is crimped
quite far back from the end of the cable. This creates an outer connector choke assembly that
makes the outer conductor longer than the center conductor.
(B) Salt Spray Survival -- My previous salt mine (the former AIL System Inc., now
EDO-Electronic Systems Group) performed a number of salt spray tests a few years ago
on crimp-connected semi-rigid cables. The results were not encouraging. In a number
of the cables the UHF or SHF VSWR changed considerably after a few cycles of the salt
spray exposure. It is hard to beat the RF bonding that a solder joint creates.
(C) Ultimate Shielding Requirement -- Arguably, the most critical requirement for an indoor
connector is that of the jumper cables on a repeater's duplexing filter. In this application
you desire the connector to provide 110 dB of shielding integrity (if you can get it). I
personally have experienced repeaters that would develop "scratchy interference" and RCVR
desensitization as the type N crimp connected jumpers were manually moved. Lloyd, NE8I also
mentioned these problems concerning silver plating. On the two occasions that I experienced
this, the problem was cured when the jumpers were replaced with well-installed conventional
type N connectors. I have been told of desperate repeater owners who used conventional type
N connectors, but modified them by soldering the internal collet assembly to the cable
braid before assembling the connector, as a way of avoiding any oxidation-caused scratchy
braid connections.
(D) Weather Proofing the Crimp -- In a conventional type N connector, the portion that
consists of the compression bond of the braid and the internal collet is all contained
within the weather-proof portion of the connector. However, in most crimp-type connectors,
the crimped portion of the cable's outer conductor is exposed to the weather. This suggests
that the crimped joint is subject to corrosion, and a subsequent poor connection. Most of us
will tape and shrink-wrap our outdoor type N connectors as a "belt and suspenders" approach
to secondary weather proofing. In the case of a crimped connector, our weather proofing of
the outer braid is a primary protection requirement.
My (Crimp) Conclusion -- If we do a really good job of installing a connector on an outdoor
coaxial cable, we are likely to use that cable for 10 to 15 years. A crimp connector is
capable of saving you a considerable amount of time during the initial installation. However,
if the crimp connector gives you trouble within the first few years of service (that's what
the salt spray tests suggest), than the time saving during the installation of a crimp
connector might really be a false economy. I'm willing to spend an extra 10 minutes installing
a connector, if it is likely to give me over 10 years of service.
Here is my challenge. Does anyone know of a well documented set of salt spray tests that were
performed on various stiles of RF coaxial cable crimp connectors? A salt spray test is a
beautiful way of artificially putting 10 years of aging into a cable assembly within a week.
Many of us live within a hundred miles of a sea shore, and this characteristic is important
to us. I'll admit that the Microwavers who live in the Mojave Desert may not have this
particular problem to worry about.
Mismatch -- Leonard, N3NGE spoke of the difficulty of sweeping a cable system that has a high
return loss connector at the beginning. Jerry, K0CQ suggested that the problem can be overcome
with a Time Domain Reflectometer (TDR), and it will even display the water that is within a
section of the cable.
I've spent a few years of my life using TDR's and I love'em. They can make RF measurements
that will amaze you. However, they are expensive, rare on the surplus market, and few colleges
even mention this wonderful instrument. That's unfortunate. A really good TDR will allow you
to inspect the integrity of your transmission line system at possibly every 1/8 inch at a
time, and it will "look through" that poor connector that's at the beginning of the cable.
There are TDR "De-Embedding Techniques" that will allow you to inspect portions of your cable
that are surrounded by some pretty significant mismatches.
There is a solution for us amateurs, it's called the Steinhelfer Technique. If you sweep the
cable, and stop at say 1,024 separate frequencies, and measure the amplitude, and phase of the
reflected power, you now have a data set that can do magic. Apply this data set to a computer
program that performs a type of Fourier Transform, and it will simulate a TDR that is far
above the performance of the one that you could afford.
We have all seen those fairly inexpensive hand held VSWR Sweeper-Plotter machines. Add a phase
measurement capability, and an RS-232 port to that machine, and you're almost there. That
modified hand held device will gather the raw data, and a PC could process the data and make
up the TDR plots. A VSWR plotter that sweeps 1 to 1,000 MHz could give you the capability of
resolving what's going on in your transmission line system every 6 inches. For most of us,
that's good enough to locate a faulty section. Sweep the data gatherer from 1 to 2,000 MHz,
and you will resolve every 3 inches, etc. It's about time that somebody offers this as a new
RF toy for our pleasure.
I'll admit that the Steinhelfer technique involves some fairly heavy mathematics. But, it can
be taken in stages, and you could share the responsibility. Just assemble an RF maven, a
mathematician, and a Computer Science major, and point them in the right direction. This
would make a fantastic Senior Project for a group of engineering students. Later, it might
even make them rich. For those who wish to study this further, see the following references:
(1) HP Application Note 62, "Time Domain Reflectometry", 1964.
(2) HP Application Note 67, "Cable Testing with Time Domain Reflectometry", October 1965.
(3) HP Application Note 75, "Selected Articles on Time Domain Reflectometry Applications", March 1966.
(4) Harry M. Crimson, "TDM: An Alternate Approach to Microwave Measurements", Microwaves, December 1975.
(5) M. Hines and H. Steinhelfer, Time Domain Oscillographic Network Analysis", IEEE MTT March 1974, pp. 276-282.
(6) P.I. Somlo, "The Locating Reflectometer", IEEE MTT, February 1972, pp. 105-112.
(7) H.E. Steinhelfer, Sr., "De-embedding the Capacitance of a Resonant Circuit Using Time Domain Reversal and Subtraction", IEEE MTT Int. Microwave Symp. Digest, 1982, pp. 354-356.
(8) H.E. Steinhelfer, "Discussing the De-Embedding Techniques Using Time Domain Analysis", IEEE Proceedings, January 1986.
(9) D.W. Hess and Victor Farr, "Time Gating of Antenna Measurements", Microwave Journal, January 1989.
(10) D.L. Holloway, "The Comparison Reflectometer", IEEE MTT, April 1967, pp. 250-259.
I'm looking forward to using this new RF Toy, so don't you guys disappoints me now!
I hope this makes you feel a little more comfortable about UHF connectors; they
are really not as poor as some think. Please feel free to correct the mistakes.
73 es Good VHF/UHF/SHF DX,
Dick K2RIW.
Grid FN30HT84DC27
