The KI7DX 6 Meter Repeater
Now with 70cm!
Time exposure view of the Salt Lake Valley at night as seen from the repeater site.
Click on the image for a larger version.
Time exposure of the Salt Lake valley as viewed from the mountaintop

About the KI7DX Repeater:
6 Meter port:
  • Output Frequency:  53.150 MHz
  • Input Frequency:  52.150 MHz
  • Subaudible Tone:  A 146.2 Hz tone is required for access.
  • Transmit power:  100 Watts
  • Electronics:  GE Mastr II Base
  • Antenna:  1/2 wave vertical dipole.
  • Signals received via the 6 meter port are identifiable by a low-pitched courtesy beep.
70cm port:
  • Output Frequency:  448.150 MHz
  • Input Frequency:  443.150 MHz
  • Subaudible tone:  A 127.3 Hz tone is required for access
  • Transmit power:  About 10 watts
  • Electronics:  GE Master II Base
  • Antenna:  DB-411 Vertical antenna
  • Signals received via the 70cm port are identifiable by a higher-pitched courtesy beep.
The 70cm and 6 meter transmitters carry the exact same audio in all cases!

Repeater Location: Farnsworth Peak (at the KSTU FOX-13 Transmitter site.)
If you have heard the WA7X Beacon, click here.

Important note about the 70cm repeater:

The 70cm port of the KI7DX repeater is back on the air - with a new frequency:  448.150 MHz (-).

Soon after installation, an interference problem was noted on the original 448.175 frequency with the -5 MHz split that rendered the 70cm repeater unusable - particularly after a new antenna was installed.  In an attempt to fix this problem, the old Farnsworth frequency of 449.100 MHz was tried, but interference (from a different source) was still present.  In both cases, there is a commercial transmitter 5 MHz above the repeater output frequency that causes an "intermod mix" that interferes with the input signal as it mixes with the repeater's output.

To verify that this was, in fact, the interference problem, a "hybrid" split of 5.925 MHz was temporarily used - an output of 449.100 MHz and input of 443.175 MHz.  This solved the problem, verifying our suspicions as to the cause of the interference.

As a permanent fix, a "frequency swap" was arranged with another repeater owner, N7HIW, who had been experiencing minor interference issue at another, distant site.  As of December 17, 2007, the repeater was back on the air using its new frequency and measurements indicate that this has, in fact, solved the interference problem - for both repeaters!

About this repeater:

This repeater first went on the air during the last week of 1999 from a valley location in Murray, Utah.  During the six months that it was there, the bugs were worked out of it and in mid-June 2000 it was placed atop the mountain.  In early August, the original groundplane quarter-wave vertical antenna was replaced with a half-wave vertical, resulting in improved transmit and receive performance.

Over the summer of 2007, the repeater was re-worked and a 70cm port was added.  Now, an a signal input on 70cm or 6 meters will be transmitted on BOTH 70cm AND 6 meters.  This dual-port repeater allows cross-band connection so that one can take advantage of the long-distance coverage of 6 meters while allowing the convenience of 70cm operation.  Having a dual-port repeater may also stimulate some interest and activity of the 6 meter side.

The UHF repeater now uses a multi-dipole vertical antenna.  Because of the antenna's location, it cannot provide coverage to the north into Weber county, owing to blockage by the mountain itself so coverage is mostly limited to Salt Lake and Utah valleys with some coverage into Tooele valley and to the west on I-80.

This repeater is currently located at the KSTU FOX 13 transmitter site (with their kind permission) and this site allows much greater coverage than the previous valley-floor location.  This is the same site as the Utah VHF Society's 146.94 repeater and the coverage of the two repeaters should be similar- but keep in mind that the lower frequency of 6 meters may allow much deeper coverage into fringe areas - provided that you have a decent antenna.
The 6 meter repeater antenna atop Farnsworth Peak
Some of the antennas at the site.  The 6 meter antenna is the vertical that can be seen on the left - it's whip being visible in the full-sized picture.  This antenna replaced a 1/4 wave groundplane vertical several years ago.
Click on the picture for a larger version.

As you might guess, the mountaintop can prove to be a very harsh environment and can be difficult to access at times.  During the winter, the only practical means of access are via helicopter, snowmobile, or snowshoe, or snowcat - but "land" access is often too dangerous due to cornices and drifts along some portions of the road.  This means that this repeater must be built to be reliable, and that its outdoor components (i.e. the antenna, feedline, etc.) must be weatherized and ruggedized in order to withstand frequent 100-plus mile-per-hour winds as well as a lot of rhime ice.

More info about the 6 meter portion fo the repeater:

As of early August, 2000 the original 1/4 wave folded vertical was replaced with a 1/2 wave vertical dipole.  This antenna is better-suited for surviving the harsh conditions that occur on-site as it has no fragile radials for the wind and ice to snap off.  It also has a bigger "aperture" (that is, it is larger) than the 1/4 wave antenna.  While the two antennas theoretically have the same amount of gain, the "apparent" gain of the full-sized 1/2 wave antenna is higher because there is simply more metal for an incoming signal to intercept.  This antenna has also been re-mounted on the tower, spacing it further away from the other two antennas:  This should allow the antenna to work better and offer less detuning due to the proximity of the other antennas.  (The tallest antenna in the picture, by the way, is that of the 146.94 repeater.)
Duplexer assembly for the 6 meter repeater showing the bandpass cavities and the notch stubs
The KI7DX 6 meter repeater - a GE Mastr II Base
Top:  The four bandpass cavities (and notch stubs) that make up the repeaters duplexer.  Although not obvious from the picture, they are mounted in the ceiling, laying on their sides.
Bottom:  The rack containing the 70cm and 6 meter repeaters.  From the top down is the 70cm repeater, 6 meter repeater, 70cm duplexer, and power supply for both.  (Yes, we know that the label on the 70cm hadn't yet been changed...)
Click on either picture for a larger version.

Another factor for many mountaintop sites is space:  Space is usually at a premium and the buildings are fairly small and often crammed with equipment - and this site is no exception.  Initially, there was some concern as to where to put four large (8 feet long, including the tuning rods) cavities.  Larry, KB7YAF (one of the engineers that works for the station) suggested that there is quite a bit of "wasted" space at the ceiling (about 15 feet up) and, sure enough, that is where we finally put the cavities.  This picture shows them installed on some Unistrut (tm) and well out of the way of any other equipment. (Good thinking, Larry, and thanks   for the help, Dan...)

Using the 6 meter port of the repeater:

This repeater is an open repeater:  That is, it is available for use by any appropriately licensed amateur.  With the recent locating of KUTV's Channel 2 VHF transmitter (54-60 MHz) just a few hundred yards away atop Farnsworth Peak, it has been necessary to activate the subaudible tone decoder to prevent the repeater from occasionally "blowing squelch noise."  Even though it a subaudible tone is required, this repeater is still open.

If you have used both 2 meters and 70 cm, you know that 70cm works better with a smaller antenna than 2 meters.  You may have also observed that in many places (such as a car) 70cm seems to "get out" better.  On 6 meters, this is even more true than for 2 meters (in comparison with 70 cm.)  Don't expect the tiny rubber duck antenna that you got with your HT to work very well.  Anyone who is a veteran of 6 meters really doesn't expect that micro-sized antenna to work well - and neither should you!  While larger antennas are highly recommended (such as a 1/4 wave vertical or a vertical dipole) you may still be pleasantly surprised as to what you can do on 6 meters.

If you have a 6 meter FM rig, QSY and give it a try...
Channel 13 tower with 70cm antenna
The main tower onsite.  The UHF repeater's antenna can be seen just below and to the left of the lower dish on the tower.  (It's the antenna mount near the dish and NOT the one that you see off to the left of the tower.)
Click on the picture for a larger version.

About The Repeater - The Radio:

In true amateur spirit, this repeater is a cooperative effort by several hams. While this repeater is Glen's (WA7X's) baby,  the radio itself (a GE Mastr II base station) has been contributed by Dave Williams, WA7GIE.  The GE is a fairly rugged radio from the early 80's that was retired from service.  It has been re-crystalled, retuned, and refurbished by John Lloyd, K7JL and appropriately modified for use as an amateur repeater.

The Duplexer:

The most expensive single item for many repeaters is the duplexer and it is this device that allows simultaneous transmission and reception from a single antenna, using the same feedline.  This duplexer does just that, and it provides a tremendous amount of rejection of other signals that may be on-site and on nearby frequencies (such as nearby TV channels 2, 4, and 5.)  The duplexer can also help protect the repeater's transmitter and receiver from lightning strikes - common occurrences atop mountains.  But getting this duplexer together posed several challenges, as you'll see...

Glen was able to procure four surplus cavities - two of them were bandpass and the other two were notch types.  Their designed tuning range is from 36 through 42 MHz - a fact that was known when they were purchased.  Worst case, it was expected that we would have had to partially disassemble the cavities, trim some of the internal pieces to raise their frequencies to the 6 meter band, and re-weld the (aluminum) cavities back together.  This would have been a lot of work, but certainly possible.  We were very pleasantly surprised, however, when we discovered that, without modification, they could just barely tune up past the frequencies to be used by the repeater:  We didn't have to disassemble them at all!

We did have another problem, however:  For repeaters that aren't located near any other transmitters on nearby frequencies, you can get away with a fairly simple notch duplexer.    Being atop a mountain at a busy site, we needed a band-pass/band-reject type of duplexer - and we did not yet have all the pieces for one.

John Lloyd, K7JL, next to two of the four cavities making up this repeater's duplexer.
John Lloyd, K7JL, standing next to two of the four cavities that make up this repeater's duplexer.  The cavity plus its tuning rod is about eight feet long!

Two types of duplexers:

In amateur radio service, two types of duplexers are most commonly found:  Band-Reject (abbreviated "BR") and Bandpass/Band-Reject (Bp/Br.)

BR types are are simply "notch" filters:  The receive side has some filters that keep the transmit signal out of the receiver (preventing overload, etc.) and the transmit side has notches tuned to the receive frequency to prevent noise (that all transmitters generate) on the receive frequency from masking weak signals. 

A Bp/Br duplexer has the functions of a BR duplexer plus additional filtering so that only frequencies around the transmit frequency get through the transmit side of the duplexer, and receive frequencies can pass through the receive side. 

Why is it so important to use a Bp/Br duplexer on a mountaintop?  A simple BR type is designed to block the repeater's receive or transmit frequency (as appropriate) but NOT block other frequencies!  That is, if you have a 70 cm amateur repeater on the same site as a 450 MHz commercial transmitter, and BR duplexer does NOTHING to keep that 450 MHz signal out of your radio.  This not only applies to other UHF signals, but that same 70 cm BR duplexer won't keep VHF out either! 

A Bp/Br duplexer allows only those frequencies that are on or "nearby" the pass frequency:  The farther the signal is away from the desired one, the better its filtering is.  This is extremely important on sites that are shared with other users! 

One of the side-benefits of a Bp/Br type of duplexer is that of lightning protection:  Since the "zap" of a lightning bolt tends to cover all frequencies, the bandpass filtering allows only a narrow range of frequencies and thereby only a very small amount of the lightning's energy.  A BR duplexer passes everything but the notch, letting most of the lightning's energies through, possibly resulting in blown-up radios and preamplifiers.

Redesigning the Duplexer:

We had just two notch cavities and two bandpass cavities.  That particular combination is not sufficient to provide adequate transmit/receive isolation on a single antenna.  If we converted the two bandpass cavities to notch cavities we might have had the necessary isolation, but the off-frequency rejection and lightning protection that we require for a mountaintop site would have been lost.  If we converted the two notch cavities to bandpass cavities, we would still lack sufficient transmit-receive isolation.

We decided that we had to build some notches.

The most critical components of a the duplexer - good bandpass filters - we already had. We decided to convert the two notch cavities to bandpass cavities and this was done by constructing simple coupling loops - the design of which we copied from the existing bandpass cavities:  This gave is a total of four bandpass cavities.

We now had our desired bandpass filters, but we needed more isolation (that is, more than the 60 db provided by the two bandpass cavities) between the transmitter and the receiver:  We needed some notch filters.  Fortunately, these are much easier to construct than bandpass cavities:  You simply need a piece of transmission line that is 1/4 wave long, a few simple components, and viola - instant notch.  We had some scraps of 1-5/8" Andrew Heliax (tm) laying around, so we decided to try making some notches with some of those pieces.

A search on the internet turned up WB5WPA's Notch Duplexer Site. (Also, see below for an alternate site with construction details.) This described how Jim, WB5WPA, built a 6 meter notch duplexer using pieces of heliax.  "Gee," we thought, "most of the design work has been done for us..."

Upon studying WB5WPA's notch design, we could see where it (the original design described as in 1999) could be changed mechanically to improve ruggedness and shielding.  The difficulty seemed to involve how to attach the connectors and other components to the top of the chunk of heliax.  Our answer was to cut a large hole in one piece of glass-epoxy circuit board material, remove about 3/8" of the jacket from one end of the hardline, slip this piece of circuit board material over the end of the hardline, and then slit, bend, and solder the end of the hardline jacket (which was copper, of course) to the circuit board material.  It was then just a matter of building a box from circuit board material where the connectors and other components could be mounted.  Additionally, we found some brass tubing that provided a very snug fit for use with the resonator coupling capacitors.  It should be noted that, as can be seen from the pictures, we added a piece of circuit board material to increase the rigidity of the box:  While in the design stages, we noted that moving the coaxial cables would cause the sides of the box to flex, slightly detuning the capacitor - a problem solved by this bracing.

KF6YB's version of the notch duplexer:

WB5WPA's web site has some details on construction of the newer versions of this duplexer, but for information describing how we assembles ours, we'll direct you to the page created by Oscar, KF6YB when he, after correspondence with us, built his own notch duplexer and documented it.  On his Second Generation 6 Meter Duplexer page he gives some details - as well as some calculators - about his second revision of his notch stubs.


After completion of one of the stubs, we evaluated it in-circuit and found that, with 1/4 wave lines on the input and output, it provided between 25 and 30 db of additional notch depth. Based on this measurement, it was decided that a total of four stubs (two stubs per half of the duplexer) were needed to provide adequate isolation - so three more were built:  Two for notching the transmitter frequency for the receive side, and two more for notching the receive frequency for the transmit side.  When the two notches were connected in series, the depth was in excess of 65 db.

Aside from the necessary frequency differences, the only other way the stubs differ from each other is by their shunt reactances.  A plain, simple notch will symmetrically cause some added attenuation for some distance above and below the notch frequency.  If a small amount of reactance is added, the attenuation curve becomes asymmetrical and losses on one side can be greatly reduced: Adding a shunt capacitor reduces the attenuation below the notch frequency and adding a shunt inductor reduces the attenuation above the notch frequency.

The results:
One of the four notch stubs added to the duplexer increase transmit/receive isolation
An internal view of the coupling section of the high-notch/low-pass stub
An internal view of the low-notch/high-pass stub
Top:  The bottom of one of the four notch stubs used to increase TX/RX isolation.
Middle:  The high-pass/low-notch stub.
Bottom:  The low-pass/high-notch stub.  The difference between the high-pass/low-notch stub and the other one is that this one uses a shunt capacitor instead of a shunt inductor.  The piece of circuit board running across the box is used to increase mechanical rigidity and prevent detuning.
Click on either image for a larger version.

All of this effort seems to have paid off.  With the bandpass cavities and notch stubs all interconnected properly, each side of the duplexer exhibits less than 2 db of insertion loss while the transmit-receive isolation has been measured at greater than 105 db - much better than we need!

Since this duplexer was installed on-site in 2000, we have not had to touch the tuning of either the bandpass cavities or the notch stubs:  The measured insertion loss was right where it was when we installed it and the isolation was too high to measure with the service monitors that we had with us at the repeater site!

Building "coax" notch duplexers for other bands:

The question is often asked "Can I build a duplexer like this for the XX meter band?"  The answer is:  Maybe.  It just so-happens that 6 meters is a pretty good fit for this sort of construction:  The frequency isn't so low as to make construction too impractical due to very large components, and it isn't so high that losses mount to intolerable levels before one gets enough isolation.

For 10 meters, the notches would nearly twice as long - too long to fit floor-to-ceiling in an average room, but maybe the pieces of coax could be coiled or bent into a "U" shape.  For 2 meters, it is uncertain if enough isolation could be obtained at 600 KHz spacing without causing too much insertion loss for construction of a single-antenna TX/RX duplexer, and whether or not temperature stability would be adequate:  For 70cm, these problems may be even worse.

We haven't experimented with this technique for any band other than 6 meters, so if you have had luck (good or bad) trying it on another band, please let us know!

The Repeater Controller:

The repeater controller is an NHRC-4 controller.  This controller is inexpensive and simple, yet powerful enough for our purposes.  (More information on this controller may be found at

Getting into the 6 meter repeater:

As was mentioned above, using 6 meters is somewhat different that 2 meters.  First of all, it is at approximately one third the frequency of 2 meters.  This means that the wavelength will be 3 times as long.  Most importantly, this means that a short rubber duck will work worse on 6 meters than a similarly-sized rubber duck will work on 2 meters.  This also means that, from inside a car or vehicle, 6 meters won't work as well as 2 meters (which, if you have been on the receiving end of someone running mobile with an in-car antenna, you know doesn't work very well, either!)
"Can't you just use a preamplifier to receive weaker signals?"

There are times when preamplifiers are helpful and there are times when they are not. 

First of all, you must understand that a preamplifier is only useful when the noise of the receiver itself is "stronger" than the noise plus the signal coming from the antenna.  It would be like trying to turn up the volume on a noisy radio station and expecting the noise not to get  louder along with the desired signal:  The noise doesn't go away, it just gets louder!

As it turns out, 6 meters is kind of noisy due to atmospheric noise (kind of like the background noise on HF, but less-so)  and our receiver can already "hear" this background noise.  Supposing that there were no atmospheric noise to speak of (such as is the case on 70cm.)  Would a preamplifier help? 

Not necessarily!  As it turns out, the hotter an object gets above absolute zero, the more "noise" it gives off.  An extreme example of this is a white-hot filament of a light bulb:  It is so hot that we can actually see the "noise" given off in the form of light. 

Now the earth is well above absolute zero (nearly 500 degrees F above absolute zero) and it tends to give off plenty of noise - but it's "cold" enough that we can't "see" it with our own eyes - but a sensitive radio can! 

As it turns out, a dipole on the planet earth at 2 meters or 70cm will receive approximately 0.07 microvolts of noise from the earth:  Remember, this entire planet is a lot hotter (270 or so degrees celcius hotter) than absolute zero.  If we were in the middle of space or if we pointed our directional antennas into space, the earth's noise would have less effect - but that's another story... 

What this means is that if the signal you are receiving is weaker than 0.07 microvolts (let's say 0.05 microvolts) it is already lost for good!  Amplifying that signal by 20 db will bring that signal from 0.05 microvolts to 0.5 microvolts - well within the sensitivity range of most any receiver, but the noise has increased to 0.7 microvolts:  Your signal is still beneath the noise!  What's worse, nearby signal that are already strong are stronger still and may overload your receiver!

The upshot of this is that 6 meters isn't really a handie-talkie/rubber duck sort of band.  Where 6 meters really shines is its range.  Let me explain:  Once you have gone through the trouble of putting up/using a half-decent 6 meter antenna (a quarter-wave whip or a J-pole, for example) you'll notice that 6 meters can carry quite a bit farther than 2 meters can.  There are several reasons for this.

The most obvious reason is that there is a decrease in what is called "apparent path loss" at the lower frequency of 6 meters, as compared with 2 meters.  Given two stations - on a 6 meters and one on 2 meters - identical stations in terms of receive sensitivity and transmit power and that they are both using quarter-wave ground plane antennas, the 6 meter station will have a signal that is nearly 10db (that's 10 times) stronger than the 2 meter station.  If you compare 6 meters to 70cm, the difference is approximately 20db (that's 100 times!)

Another major reason for 6 meters carrying farther is that it propagates differently.  In many cases it can seem to get over mountains better (using various propagation modes such as knife-edge and diffraction.)  Finally, it is much less affected by absorption of foliage than 2 meters and, especially, 70cm.

These improvements come at a price, however:  6 meter antennas are quite a bit larger than their 2 meter counterparts.  6 meter operation is also more likely to cause TVI (Television Interference) than either 2 meters or 70 cm (especially to channel 2.)  6 meters is also more likely to be affected by powerline noise than 2 meters on receive.  (As you should know, powerline noise that is getting into your receiver locally is not  transmitted to the other end.)

Another factor that you may (or may not) consider to be a disadvantage is that 6 meters has frequent band openings.  When 6 meters is open, it is much like 10 meters in that low power can allow you to work all over.  Openings on 6 meters are less frequent that those on 10 meters and there are fewer people on the band.
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Who is KI7DX, anyway?

KI7DX is the call held by Janet Worthington, Glen's better half.  Why use her call?  Could you resist using a call like KI7DX?

Whilest you are here, look at these other pages:

For more information about the KI7DX repeater, you can send Glen some email.

This page last updated on 3 April, 2008

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