The AM Forum

A yagi and a 1/4 wave vertical attached by a piece of coax at UHF frequencies.  The yagi is outside the building and the vertical is inside the building where repeater coverage is spotty.

My boss pulled out this contraption and asked me to test it at a site where we have bad coverage for an NXDN system.  I immediately started laughing because I thought it was a joke.  

We have two of these systems installed at two different sites.  One supposedly works great and the other doesn't according to end users.  

I don't think these work.  Any other opinions?

Jon

If one had a good spectrum analyzer, with good dynamic range, a fairly simple test could be performed.  With a test antenna, preferably the same as that used on the HTs in the NXDN system, measure the signal level at various indoor locations from the exterior signal as "repeated" via the passive repeater lashup.
Then, disconnect the indoor antenna, and repeat the measurements for comparison, keeping good records on a map.  It should be possible to map out areas where there was a signal enhancement.... or not. And this may well be a multiple variable situation, dependent of the location of both indoor and outdoor transmit/receive locations.
In absence of a spectrum analyzer, one could do a similar quick and dirty experiment by walking around with a handheld receiver and trying to determine locations of improvement with the indoor antenna connected...or not.  Obviously, the cheap and cheerful methodology is more qualitative than quantitative.  Just because lab quality test equipment may not be available is no reason to not cobble up a quick experiment, as long as we understand the limitations of the results.  Depending on how long you wanted to milk the project, you could avoid real work for days!
The old microwave "flyswatter" antenna arrangement is the closest thing to a "passive repeater" that I've had any experience with.  It worked, but it was excited by a high gain parabolic dish, only a few hundred feet down the tower. 
Keep us posted.

I did a lashup test and reported that it wouldn't work, and was pulled off the project and replaced by a believer.  My test was simple.  I just walked around the cubicles with the same Kenwood portable that they used, with and without the passive repeater.  I noticed no difference.  They did go through with it anyway and put the yagi on the roof with a mag mount antenna just above the secretaries on the ceiling. 

...and the end users still aren't happy.

Note:  My test setup didn't include Kachina dolls, holy water or Indian seeing-eye gods.  That's probably what I was missing.

Jon

Yer negative thoughts interupted the aura of the area.

You need the re edjucation camp.


KLC


A true beleiverrr

Kool-Aid comes in lots of delightful flavors, many of which are fortified with preconceptions.  Over the years I've been fortunate to have only one boss who drank technical Kool-Aid and expected the rest of us to partake as well.  I outlasted him, though.
There's no substitute for decent test equipment, planned experimentation and logging it all in a laboratory notebook.  I had another boss who was a stickler for lab notebooks, and I quickly learned to keep mine at hand and updated.  It's too bad that I haven't always been as diligent in documenting my ham radio projects.

Well... the test would be to find a signal that can not be received inside, then receive it with the Yagi connected directly, and then
see if it could be heard inside with the vertical connected.

Seems to me that the signal from the inside vertical will be somewhat less than the signal at the Yagi or at the end of the coax
from the Yagi, since it is dispersed in an omnidirectional manner - assuming it is actually tuned and represents a low SWR.
IF it's a high SWR, then it looks like an attenuator for all intents and purposes... keep in mind that ALL of the signal in the coax is
focused into the front end of the receiver...

I presume they want this to work as a two-way system?

I think if it is going to get even close to a unity gain thing they'll need a very directional antenna downstairs.

Passive repeaters work IF.

The incoming signal to the yagi outside is sufficient.  The coax is low loss if it is long, connectors are proper, and

 the vertical has radials, then mounted upside down. 

You don't say anything about parameters so it is difficult to say yes or no, but I have seen them work well.

This is meant to be a 2-way system. The mag mount 1/4 wave vertical is mounted upside down on the ceiling. The lowered ceiling cross beams serve as the metal mount. The signal coming in from the repeater on the roof is about -90dbm tops as measured by an Anritsu analyzer and a rubber duck. The yagi is 5 elements. There is also a tall building close by that blocks the path to the repeater.

Jon

Tell them to stop being cheap and buy a BDA for uhf.. Do it right.. Do it once..
Jon, Who's NXDN network are they riding on?.  There is a VHF Nxdn trunk that a I know of.. The company that I work for has a 800mhz trunked  nxdn.

This is Kenwood's NXDN system that we're sharing with the school system. There are two master sites, and we're debating adding a third. This one department is having a problem in their building with coverage. We've looked at getting a BDA, but the quote came in at $75k. That's more than the department paid for all their radios combined.

I recommended that each secretary put a mobile on her desk with an external antenna, but that didn't fly either.

Jon

Had to read the posts a few times to understand what you're trying to do.  You're picking up a repeater signal with the yagi and then expecting the vertical to re-radiate the signal inside the building.  And then visa-versa.  Seems like a great idea except for one thing,  the loss of signal in the connecting cable.  Probably less signal being re-radiated by the vertical than the signal reaching the same location directly.  This concept might be made to work if you use some sort of amplification to overcome the line loss and give some positive signal level inside the building.

Of course this results in only a one way system,  outside to inside.  So, you use a second antenna pair with amplification going the other way, inside  to outside.  Some thought has to be given to the careful placement of antennas.  Too much amplification could create feedback problems.  

You should be able to make it work.

Another thing to consider is phase shift.  Signal being re-radiated by the vertical will be out of phase to some degree due to the VOP of the cable.  This could cause even more problems.  To overcome this possibility, the amplified signal would have to be much stronger than the directly received signal.

Anyway, it seems like an interesting problem to work on.

Fred

This was just a quick/cheap attempt to see if it would work.  And apparently the way we've done it doesn't work. 

Going further with amplification and two unidirectional paths is beyond the scope I think of any current employee.  We got rid of our Engineer's position a couple years ago, so we'd have to contract out that work.  I'm at the tech level...

Jon

Jon, You gave them the right recommendation... A base in the main office is the correct way to do it.. The only other way ( besides a BDA ) is adding a additional mountain top site that back fills into the area you need. To cover the entire valley (phoenix )is much more challenging then people think.. The 800NXDN uses, Whitetanks, ShawBute, Userypass, Mt Ord, Towers Mtn and ( Sacaton  yet to be built ). Plenty of saturation is key to digital 2 way.  I know some of the logistics behind it all. It takes massive amounts of $. Then you have to network it all to make it play seamlessly.

Im sure your dealing with one of the Kenwood dealers. So in efforts not to step on anybodies toes, if you need any more info, contact me via PM.

Sam

I've read of a similar lashup used to create an isocoupler to cross the base of a hot AM broadcast tower.  It consisted of three Scala paraflector type antennas with one antenna at the top of the tower and another at the base of the tower connected to each other by coax. The third antenna was mounted on a wooden post set in the ground with its driven elements pointed at and separated by only a few inches from the antenna elements at the tower base.  It supposedly worked FB in coupling the RF from a 950 MHz STL transmitter into the transmit antenna at the top.

73,
Jim
WA3AJM

When I lived in Marshlands / Gaines, N. Pa some years ago (as wa3ypi) I thought about an 11 element Cushcrft 2 meter at the top of Elephant Mountain hooked about 100 ft.away to another one pointing down slope to my house in the Elk Run Valley.  This because I couldn't reliablly hit the Mansfield repeater.

Not on my property but on state land, so worried the lash up would be vandalized by probably hunters . I never really got To do this trial. not sure it would make much difference. I'd have to verify re-direction and make sure I was at break over angle up there, hence might need longer coax, loss vs.coax distance, etc.
 Adding an amplifier to my IC 22s down at the house solved most of the problem.

In the case of the office bldg. presented, I agree with installing the office system properly.  
I'd've just installedd a building repeater. Hey, whats a few more watts?  Actually, I've been in similar personnel situations where you install a cheap system to placate the secretaries and lowly engineering types, then placebo effect helps quell the revolution for awhile. ;D. If real engineers call your bluff you simply say, "ok, you're probably right. Submit it in the budget for next year."  ;D

I got interested in this about a dozen years back for a friend on the far side of a hill from the repeater he needed to work and also thought about a pair of beams and some good coax over the peak. My reading indicated that the military used rhombics on either side of some of the peaks in Korea during that war, tied together with open wire line to get over the ridges when knife edge diffraction wasn't enough. Never saw anything about how well it worked though.

No way!! Speaking as an experienced hunter I can assure you that we would not do such a thing! For one thing, we are too drunk to walk! So moving from our blind would make noise and we all know that we will shoot at anything that makes noise in the woods. The other thing is that we are too dumb to understand what those wires and metal pieces are, possibly thinking it's a trap or some part of a UFO? So simply stated, anything we don't understand will be undisturbed less we violate our code of the unknown? Strange things happen in the woods but this ain't one of them.

It looks like we had a bad repeater TX antenna. The one in service was 15 years old. There was another UHF antenna unused about 20 feet down the tower. When we switched to that, signals improved inside the building by 10db.

So, all's well that ends well?

Jon

Jon
Fisher Wireless building on Shawbutte?

We're renting a site on Shaw Butte, but I'm not sure who owns the building.

Jon

30+ years in two way for GE and Motorola and I've seen it all, including the passive repeater fantasy. Never seen it work. The shop I work for is in biz to make money. We sell and install BDA's. Properly deployed they work very well. How do you like NXDN? Is the audio any better than Mototrbo? Trbo has worst audio I ever heard in my life. Customers routinely reject it and ask to go back to analog.

NXDN sounds great but I guess its all about preference. I know some guys are nuts for trbo..  Ive heard both and would take NXDN any day but there is a lot of trbo out there..


Jon
The building I'm talking about is the first one on the left when you get to the T. Straight you go to the broadcast site. Left you turn and head towards the sites.. Fisher is the one immediately to the left..

I haven't been up there enough to remember all the buildings.  Ours has a City of Mesa placard on it.  Our tower is smaller on the west side of the hill.  

Our customers seem to like the audio.  They say it's "loud".  My guess is that the Kenwood radios make their volume knobs go to 11 instead of 10.

Jon

Behold! The passive repeater god.

Jon

Jon
Ah Ok. I know which one your talking about now.. Im glad to hear you got it working. NXDN sounds great. There is also audio adjustments in the radio programming. The high end 5000 series radios are real cool. You can program all kinds of bells and whistles in those.

I just finished building an even bigger one! 100 feet of LDF4 coax with two UHF verticals and a Yagi pointed at the radio master site.

Yeah, it still doesn’t work. I think it’s a matter of critical mass that we haven’t hit yet. Maybe phased arrays?

Jon

Time to stop playing in the amateur world and go to the professional world. Cheap fast solutions aren’t going to get you anywhere. You want converage inside a building. Get the city to pony up and contact EMR. If you don’t want to handle it yourself. PM me. I can put you in contact with the people that can make it happen.

What you are trying, passive repeater, helps when a totally enclosed metal building is the problem and the frequencies are UHF+. On a cellphone, made the difference of the phone ringing and no one there, to the connection actually going through. The coaxial cable was nothing special but only 1 FT long. No idolatry was employed.

That's the problem I see.  Everything on the web relating to passive repeaters is anecdotal.  I haven't found any industry or government studies on the subject, but rather someone's story saying, "It works."  This is my fourth time dealing with this issue, and not once has it worked.  Using a spectrum analyzer, I see no increase in signal strength.  Certainly if it were sound science, wouldn't MFJ have a product for it by now?  They have a knick knack for everything. 

Every time I mention that these don't work, or joke that they're voodoo technology, it's as though I'm insulting someone's religion (which is in itself a series of anecdotes). 

Jon

I believe I am lucky and nothing else. No science was used therefore I can't possibly be insulted.

The people in charge who want their serious radios to work need to cough up some $, pay someone, and use commercial guaranteed-to-work stuff that runs off 120V outlets.

I am astonished that you actually went as far as using 100' of coax. any coax.

I just performed a test, by accident yesterday.  I brought in a 9-element quad antenna that I want to experiment with for satellites, and among the tests I did was a front/back ratio test.  I put a signal generator on 437 MHz at -50db on my desk with a little UHF vertical antenna.  I then stood 10' away with my quad plugged into an Anritsu spectrum analyzer.  

Pointing my quad directly at the antenna, received signal was -82dbm and 180 degrees away was -97dbm.

Coincidentally, one of the readings from the passive repeater test that I installed was that the incoming master site signal which was -50dbm, from the yagi mounted outside. Again, this signal was supposed to passively repeat by the little UHF vertical antenna which it was connected to inside the building.

To use the numbers that I used at my bench with my homebrew satellite antenna, an end user in the building would need to point a 9-element quad 10 feet away directly at that little antenna to get a -82dbm signal.  From my measurements in the room without the passive repeater, background signal alone from the radio master site was -78dbm.

I then plugged in the passive repeater and found absolutely no difference in background room signal strength (obviously, because I didn't have a moon bounce array of beams pointed at that little UHF vertical antenna).

Jon

If you are expecting to see gain through the antennas, it won't be there.  From what I read that is what you are looking for.  You will see the same signal level at the output minus the losses of the coax and connectors that you put in on the other end.

The entire premise of passive repeaters that are intended to provide indoor coverage is to reduce the effect of building losses enough so that the net signal into the building is enough to provide reliable communications. You don't need to get the same level indoors as you have outdoors, you need only get enough signal to meet your performance requirements.

For example, let's say you have a radio with a working sensitivity of -120 dBm, and you want a 10 dB margin to account for a fading environment, so your design threshold sensitivity is -110 dBm. Your building presents a 30 dB loss to the desired signal. The outdoor signal at the location of where you would install the donor antenna is -95 dBm, not strong enough to set your hair on fire but plenty strong enough for outdoor coverage with 15 dB over your design margin. However, the signal inside your -30 dB building would be only -125 dBm, so you're now below the radio's working sensitivity and you can't communicate reliably, if at all. But if you select a pair of antennas and cable such that the resulting loss through your passive system is less than 15 dB you're gold, but only right at the location of the indoor antenna. The indoor signal level will fall off rapidly due to free space loss, and losses presented by indoor obstructions like walls and such.

Passive repeaters can work, but as you can see they have very, very limited applications and generally cover small indoor spaces. They are not fantasy, but they fail because they're seldom the right choice for the application. In today's marketplace with the availability of small and relatively low cost BDAs, active devices are definitely the way to go. You just have to choose one wisely, one that will not only provide sufficient gain, but one that will work in your particular RF environment (overload and interference from unwanted signals, etc.).

Exactly. You can pay your tithes, but you'll probably get more bang for your buck going to Las Vegas.

Jon

Jon
Contact Procomm. He’s one of your Kenwood dealers.

Such things have been around for the cell bands for years. An amplifier is always in the mix though to amplify the received signal and provide isolation between the input and outputs. Wilson makes some pretty good one. Should be able to do the same with your freq/requirement.