Passive VHF Repeaters

Discussion in 'General Technical Questions and Answers' started by K9ZMD, Oct 19, 2003.

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  1. K9ZMD

    K9ZMD Premium Subscriber QRZ Page

    Sure would like the benefit of anyone's experience with something I've heard referred to as "passive repeaters" for VHF simplex communications.

        As described to me, the purpose was to channel some usable signal between two fixed points that are within normal FM simplex range, but totally shadowed from one another by the terrain.  The method was to install two beams at a location that gets a good signal from both fixed points.  With one beam aligned for max signal from one fixed point, and the second beam aligned for max signal from the other fixed point, connecting the driven elements with low transmission line allows simplex operation between the two (otherwise shadowed) fixed points.  

        It sounds like a cheap, lightweight, and very simple method to fill RF dead spots when events like bicycle runs, walk-a-thons, and back road car rally's are conducted in very hilly terrain.  On one hand, it sounds too good to be true.  On the other hand, if simple communication is the goal - not a noise-free conversation - then even a scratchy signal is better than none.

        Although repeaters, cross-band repeaters, and simplex repeaters might also be used, they each may have some drawbacks under the described circumstances.  

        A Google search yielded several hits suggesting that passive repeaters can work at microwave frequencies (however, most seemed to "aim" at stealing high speed Internet access).  Other hits talked about less ambitious use of passive repeaters, like improving VHF signals inside metal buildings for pagers & hand-helds.  

        Again, I'm hoping for the voices of experience here, not opinions. No matter what mode or bands were used, has anyone actually used, or tried to use, something like this "passive repeater" (regardless of what you called it), for simplex communication?  What sort of results did you get?  How did you set it up?  What capabilities & limitations did you learn about when you put it to use?  Did you learn some things that didn't work & should be avoided?
  2. Guest

    Guest Guest

    At the Edmonds, WA ferry terminal they have two yagis on a pole at a right angle to each other to get the signal from the HTs to turn the bend from the metal enclosed car deck to the ticket booths up the road. They run the HTs at 151MHz so it looks like it would work ok for 2m work. I'm thinking that the length of the coax between the two antennas may be important. The one they have looks to be about 1/4 wave.
  3. Guest

    Guest Guest

    Interesting subject, hope to see some good answers here. Here, in the Rocky Mountains, there are some very large (look like billboards) reflectors on mountain tops. Various services aim their microwave dishes at them for a bounce. Probably not to practical for 2meters.

  4. K9STH

    K9STH Ham Member QRZ Page

    Although I have not done this on the amateur 2 meter band, I have had some success with the commercial 450 MHz band and the Part 15 900 MHz band. On 450 MHz the system was used in the open pit "strip" mines for lignite (a soft form of coal). One yagi was pointed at the repeater and the other was "angled" down into the pit. This helped considerably since about half of the pit was shadowed by the edge.

    For 900 MHz "spread spectrum" use I have used two yagis at various angles to each other in a "fair" number of situations. What this was for is an inventory system wherein bar codes are read by handheld readers that also incorporate a transmitter (you will often see these same type of units being used at WalMart, Costco, etc.). By adding passive repeaters the "normal" range which is usually up to about 500 yards (if everything is out in the open) could be extended up to almost a 1/2 mile. Also, a passive repeater system was used to get around the corner of large buildings, etc. There are a couple of these systems that have up to as many as 5 passive repeaters including one that actually uses a passive repeater feeding another passive repeater. The power output of the main r.f. station is 2 watts and the handheld units about 500 mW (0.5 watts). Before I left TXU, we had about 40 of these types of stations set up all over the State of Texas.

    Also used cheap "CB" type RG58/U as "Radiax" (registered trademark of the Andrew Corporation) run all over the large warehouses to eliminate "dead" spots. This type of coax has under 50 percent shield. Frankly, it is pretty lousey as coax even on 160 meters! But, when used as "Radiax" it works great.

    Glen, K9STH
  5. Guest

    Guest Guest

    So Glen, does the length of the coax between the yagis matter? Does it being a fraction/multiple of the wavelength help, hinder or just not matter much?
  6. Guest

    Guest Guest

    If both antennas are a good match to the coax (52 ohms) then the only downside to a long length is the attenuation in the cable, won't be much for a few feet. ( I have only two feet, how many do you have?). TOM K8ERV
  7. K9STH

    K9STH Ham Member QRZ Page

    Basically, the shorter the better! On the 900 MHz system I used less than one foot.

    Glen, K9STH
  8. ZL1BPU

    ZL1BPU Ham Member QRZ Page

    One point that has not been mentioned in this thread - I believe it is possible to amplify the signal in both directions. I recall a 2m repeater extender somewhere here that use two sets of yagi antennas on either side of the top of a hill, each with a preamplifier and coax across to the other side of the hill. This provided some gain in both directions at the cost of more complexity.

    I don't know what gain was possible while still maintaining stability, nor do I remember much of the details, but I recall it gave the locals in the shadow access to a repeater some 80km away.

    Does anybody have experience of "amplified passive repeaters", if I may call them that? I'd be interested to know, as it sounds like a simple enhancement.


    Murray ZL1BPU
  9. Guest

    Guest Guest

    ZMD, I've used passive repeaters over the years, on 150 and 450 mHz. Some worked well, some did not. It seemed the best results were had when the donor signal was very strong, and the dead area was very dead, like on the inside of a building, where no signal could get through the concrete walls. But where the donor was only mediocre, and the dead area was just weak, it was hard to notice any difference.

    By the way, if it works, you will be able to use it for simplex and repeater operation. In fact, it will work better if you use a very high gain antenna pointed at a fixed point donor, like a repeater.

    In one case, a 152 mHz paging xmtr on the roof of 11 story building could not be received by pagers in the 2nd sub basement. I put a 1/4 wave whip on the top and bottom of an unused piece of 7/8" hardline that ran from the top floor to the sub basement, and the lower whip radiated enough signal to call the pagers reliably. Still a weak signal, but useable.

    You probably won't notice amazing results, because at best you can probably only add 5 or 10 dB, without any active amplification.

    73, KM3K
  10. KW7DSP

    KW7DSP XML Subscriber QRZ Page


    I spent my early teen years in Palmdale, CA. I used to live at the Stage Stop and Butterfield Hotel out on the Sierra Hwy South and West of Lake Palmdale. My dad owned the Alpine Springs (Harold) Bar where the Mobile Park is, in the early sixty's. We also had the ranch above Rancho Courson, back when the area was undeveloped. There were 6 houses on one street at the very dead end of Ave S and our ranch (3 houses). The Antelope Valley Freeway off ramp at Ave S, goes right through our house. Go Falcons.

    In 1973 I lived in Goshen Oregon. My friend lived over the hill off 30th avenue in Eugene. We were only five miles apart, but the hill kept us from using the CB 50% of the time. We installed a 5 element beam pair as a passive repeater above Lane Community College on the hill by an old homestead. From that point on we used it 100% of the time for many years. Problem solved.
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