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K7POC
05-07-2011, 05:52 AM
I'm involved with my radio club's solar powered vhf repeater that sits at 5900ft. The target area for use in around 500-1000 feet. Down low the repeater is hard to hear but get up to 1k-1800ft and the darn thing works perfectly. The receive is great no matter what. Our receive antenna is ether a Diamond x20 or x50. I can't remember witch. The TX antenna is a used two piece antenna that's about six feet tall. Hints I don't know the gain or pattern on it. Right now the receive is much much much better than the transmit. There are several causes but the one I'm working on right now is the TX antenna. Anyway what type of antenna/brand/model might I try in order to push the RF down into the valleys better? I was thinking about going with a zero gain antenna to get the RF pattern down lower. Any info would be much appreciated! 73's

K7POC

K7JEM
05-07-2011, 06:16 AM
I'm involved with my radio club's solar powered vhf repeater that sits at 5900ft. The target area for use in around 500-1000 feet. Down low the repeater is hard to hear but get up to 1k-1800ft and the darn thing works perfectly. The receive is great no matter what. Our receive antenna is ether a Diamond x20 or x50. I can't remember witch. The TX antenna is a used two piece antenna that's about six feet tall. Hints I don't know the gain or pattern on it. Right now the receive is much much much better than the transmit. There are several causes but the one I'm working on right now is the TX antenna. Anyway what type of antenna/brand/model might I try in order to push the RF down into the valleys better? I was thinking about going with a zero gain antenna to get the RF pattern down lower. Any info would be much appreciated! 73's

K7POC

OK, there are a couple of things here. One, if your RX is OK, then that type of antenna must be working. Why not put the same antenna on the TX? Two, could your TX problem be caused by low TX power, instead of antenna? Three, for the most part, something like a DB224 is going to be your best bet. Four, how far from the peak is your area of desired coverage? From that, you can calculate the angle of downtilt you need, but I suspect that really isn't the issue here, if the RX antenna is working fine to that area.

Joe

K7POC
05-07-2011, 07:12 AM
Part of the great RX is that the repeater has a $200 preamp on it and there is a super low noise floor at the location. We are putting about 30w into the coax. Just got some more solar panels, so we may up the power. The repeater is 30-40 miles away from the target area. Also our RX antenna is up much higher than our TX. I think someone has another antenna identical to the current RX antenna. We are going to try it as a TX antenna this summer. Actually I was thinking about just swapping the TX and RX antennas. Our TX is the lowest antenna and the RX is the highest white one that is being hooked up.
http://a4.l3-images.myspacecdn.com/images02/144/8112cde0d727495e92b72d1c5f00fe73/l.jpg

N0AZZ
05-07-2011, 10:48 AM
Try a DB-224 for your repeater frequency it is about the best and maybe 50w is what you need.

NA0AA
05-08-2011, 04:34 AM
IMHO, it's better for a repeater to hear well, but it's probably an endless arguement.

If you are working rather distant area, I'm guessing most of your mobiles are 50 watts or so, which means you probably need to match that with your transmitter. Sadly.

Sadly, the fiberglass radome antennas don't last forever, particularly in mountaintop repeater service, so you may find the problem solved with a simple antenna swap. I recommend painting the X-50 before you install it though, it will greatly improve it's lifespan to rough it up and give it a few coats of paint. Dark will also help it shed ice.

Another consideration is how easy/hard access to the antenna IS. Most of our repeaters antennas are hard to get to, so using a first rate, all metal commercial antenna is worth the small initial cost when compared to the costs to get up and replace a failed antenna.

W8JI
05-08-2011, 12:13 PM
I think you need to apply some geometry and reason through this, instead of just guessing. You are already making a fatal mistake is assuming the down-tilt angle is a problem and the antenna has to radiate with more down-tilt. That's sure to bring out some nonsense stories from two-way radio guys who claim to not hear base stations 100 feet from antennas from pattern null issues.

Also, the antenna you have almost certainly has a terrible pattern. It can easily have a null, or up-tilt, or anything else. The pattern will even vary with mounting structure and feeder length, because a similar antenna I have has that problem in spades. You could easily have a low angle down-tilt null that other antennas will not have.

So do the geometry first. Look at how far stations are from the repeater, and what the angles are. Then you can look at KNOWN patterns of good reliable repeater antennas designed for commercial service, and see if you really need down-tilt.

Most VHF and UHF Ham antennas do not have the pattern or gain predicted or guesstimated because they are sloppy designs. They often have high levels of common mode currents exciting the supporting structure and feedline. They often are not even modeled properly, and are virtually never measured.

That's one of the reasons a commercial antenna with the same published gain as a "Ham antenna" is twice as long and ten times the cost, but the worse thing is the unpredictability of elevation pattern in Ham antennas.

73 Tom

M0GVZ
05-08-2011, 12:27 PM
Get a Jaybeam. Amphenol Jaybeam (http://www.amphenol-jaybeam.com/base-station-antennas.php) are a manufacturer of commercial VHF-microwave antennas and the chances any repeater antenna you see on a commercial or cellular tower is a Jaybeam.

Bad news is that because they are real proper antennas and not mickey mouse amateur ones like the X-50, they cost several times what a X-50 does. Good news is that they give discounts to the amateur radio community. All you do is phone the company up and tell them exactly what frequency you need the antenna making up for, what radiation pattern you want, both elevation and azimuth, what type you want (monopole vertical/yagi etc) and that you're an amateur radio repeater group. They will make a custom one off antenna that will be spot on on that frequency with a radiation pattern you want, or as near as technically possible, and outperform anything you can buy at a ham emporium.

Now over to the other side of the problem which pretty much everyone in this thread seems to have forgotten about. With an elevation like that, the last thing you want to be using as the station trying to ping/RX the repeater is an antenna with a low take-off angle. Sadly most users will be using 5/8 wave or higher with very low take-off when actually a 1/4 wave with a higher take off angle would be far better....

M0GVZ
05-08-2011, 01:02 PM
Just to add to the above post. Look at their vertical dipole range. You get a fair whack of signal being transmitted BELOW the horizontal in the elevation plane compared to a traditional vertical which will have virtually no signal being transmitted below the horiztonal in the elevation plane. All their antennas have the radiation patterns in the antenna data so download it, make a rough scale drawing of the elevation of repeater and the area it covers and then see how the vertical dipole elevation pattern works better than an equivalent "normal" vertical.

You might even be as well just building your own vertical dipole. People forget about the vertical dipole - 7dBi of gain, maximum gain in the 0 degree elevation and a large amount of RF power radiated up to 25-30 degrees above AND below the horizontal line. A vertical dipole for non-DX application has massive advantages over all other verticals when the TX/RX is significantly higher than the other stations, especially like yours is.

WB2WIK
05-08-2011, 07:59 PM
I think Tom (JI) is pretty much on target.

But you didn't say how high the TX antenna is above ground, or the RX antenna, other than the TX is "much lower."

When you're on a mountaintop, sometimes the antenna has to be fairly high above ground to produce a signal that isn't blocked by the mountain it's on. We have repeaters all over southern CA that are up 5000-6000-7000+ feet and the decent ones all have antennas on tall towers above that, to give them a real view of the service areas.

Using separate TX/RX antennas is almost always a problem. A DB224 or similar, very well elevated above ground, with a duplexer to allow the use of just one antenna for both TX/RX, is bound to work substantially better than what you've described.

I was never successful using Diamond/Comet/similar cheapie antennas on mountaintops, as they really can't handle severe weather which is almost always found on mountaintops. I had a Diamond collinear vertical on only an 80' tower (not all that high) on one of the local mountaintops here and it lasted less than one year before it snapped in half. I didn't see that happen, so not sure if it was a microburst or just a big bird flew into it, but whatever, they just don't last "up there." Replaced it with an RFS Super Stationmaster and never had another problem over the next 15 years.

KM1H
05-08-2011, 09:35 PM
I had 2 of those Diamond POS snap in half here in under a year and they werent very good performers either. Replaced them with a pair of surplus ~ 22' commercial band stick for 2M and an 8 bay folded dipole commercial for 440. Those kick some serious butt during a contest with 400W (PP 4-125A's FAA amp) on 2 and 250W (HB 4CX250B cavity) on 440. Ive worked into MD on 440 simplex and tropo into GA on 2.

Carl