800 Mhz yagi eliminates multipath on P25

Status
Not open for further replies.

fluke281

Member
Premium Subscriber
Joined
Dec 24, 2010
Messages
20
Location
Toledo, Ohio
In my area, there is a P25 system which continues to expand and has 13 towers within my county alone. The multipath from overlapping signals on the same frequency makes reception annoying and often unintelligible in mobile or portable listening. I have tried various adjustments to my Whistler and Uniden scanners without much results and, for base operation, I tried various outdoor and indoor antennas including a paper clip. None of this accomplished much to eliminate the multipath. I took an old UHF antenna and cut the driven element to 800 Mhz and mounted it vertically.. This helped a fair amount. Then I bought a 7 element Chinese UHF yagi for 11 bucks on a very popular auction site. I mounted the antenna at 17 feet and aimed it at a tower 1.5 miles away. The reduction in multipath is unbelievable.Signals are very intelligible now. Signal strength is not the issue. There is about 10 Db gain but the improvement in directivity and rejection of signals from the rear of the antenna makes all of the difference. You can certainly buy better antennas. The director elements of this unit are not riveted and the rear antenna mount is made of bendable aluminum. I used a u bolt mount that actually bites the mast immediately below this antenna for stability. You can certainly build your own antenna using PVC pipe and coat hangers and add more elements for greater directivity.
 
Joined
May 1, 2017
Messages
95
Location
Basement Dweller, Huntley IL
Homebrew 800 Mhz Yagi works Well

I have made 3 home-built 800 Mhz Yagi antennas from PVC and #12 AWG wire and they do work well on IL Starcom 21. But I had to really fiddle with & tweak the driven element spacing to the closest director to really peak in the signal strength (2x more). A few millimeters off either way here and I lost a lot of signal. When I hit the optimal spot, Wow. (BCD396XT Band Scope mode used as spec analyzer at antenna) But the D.E. rods have to be in true perfect alignment with the closest director and reflector, otherwise more signal loss. I even used a digital micrometer to size the rods and measure spacing out and then "eyeball" the alignment and glue it. Then adjust again for best signals, re glue it, peak it in and mount with some ties/clamps. Next I tried 1/4" copper tubing and this works out much better as it stays straight once you line up the antenna. I used a 75 ohm to 300 ohm tv xfmer and soldered the leads to the split D.E. element. The antenna is used vertically. My next one will be on a wood rail, as drilling PVC is easy, but the alignment side to side is never quite right. The "Best" design I tried had (5 total rods), three directors, the D.E. rod, and a back reflector. I have a ten rod longer Yagi antenna with #12 AWG wire and it doesn't seem much better than that one, just narrower beamwidth. Millimeters and 54 yr old eyes don't mix well.

I can also get 99% RF quality signals easy on the Control Channels all the time and yet...the voice channels themselves still fade at certain times of day. But, the 800 Mhz Yagi antenna gives me optimum signal levels on every one of those voice frequency channels for that system/tower set once you get it peaked in. My towers are scattered around too (for one System Site only) and that's the monkey wrench in this whole thing. The McHenry site is much closer to me has a better control channel signal but the voice channels are crap, the Kane tower site system is further, yet both the control channel signals and the voice channels are at 99% with any antenna. Even in a basement on the RS 800 Mhz antenna.

I swear that the signal fades on APCO P25 800 Mhz are due to atmospheric conditions, the differing thermal patterns over varied land areas, the wind, the dew point also. I have multiple antennas to use, even two 7' foot 800 Mhz tuned home brew collinears in PVC tube on signal boosters. Work excellent on 155/460 Mhz Rx too. But at times, the RF signal quality on 800 Mhz P25 (voice channels) is Crap on all the antennas. Fades, not Simulcast Distortion (or is that causing these fades?) Then I stick the "paperclip" into the ant jack and the signal is just perfect again. Arrrrgh!
 

fluke281

Member
Premium Subscriber
Joined
Dec 24, 2010
Messages
20
Location
Toledo, Ohio
800 Mhz yagi performance

Congratulations with your success on this antenna. I built a 450 unit and, fortunately, my Comet antenna analyzer works there so that I was able to tweak the match with the coax. It is indeed very touchy and is also dependent on whether 50 or 75 ohm coax is used. The copper tubing widens the bandwidth and the feed on my cheap yagi is a folded dipole, the same type of directed element used in commercial and amateur (Sinclair) vertical UHF gain phased folded dipole units. I am going to order another yagi just to dissect the feed which must be coaxial. My 900 Mhz ham beam also uses this type of feed and I believe it gives greater bandwidth since the separation on 900 for repeaters is 25 Mhz. A friend suggested a great trick for adjusting the length of the driven element: use copper tubing which is a bit too short. Then take tiny sheet metal screws and insert them in the ends while observing performance.
I also agree with your observation that atmospherics affects 800. A friend of mine who contributes to RR and has a feed here has observed the same thing many times.
 

W8RMH

Feed Provider Since 2012
Joined
Jan 4, 2009
Messages
8,112
Location
Grove City, OH (A Bearcat not a Buckeye)
I use a MAXRAD 800 Yagi on a torch light pole in my 1 story apt, 6' above the floor, aimed at a tower about 8 miles away (flat). It has worked great for the multipath issues that were degrading my feed and make listening to this system a challenge. Click the link in my signature to listen.
 

kruser

Active Member
Premium Subscriber
Joined
Nov 25, 2007
Messages
4,089
Location
West St Louis Cnty, MO
I use a MAXRAD 800 Yagi on a torch light pole in my 1 story apt, 6' above the floor, aimed at a tower about 8 miles away (flat). It has worked great for the multipath issues that were degrading my feed and make listening to this system a challenge. Click the link in my signature to listen.
Same here on the brand but mine is the 12 element version and uses an adjustable gamma match for tuning it.
I have it on the roof of a 3 story apartment building that sits pretty compared to the surrounding buildings.
Mine is also on a cheap TV rotor that I modified with a gear and 10 turn pot for accurate direction feedback.

That mod was king of a bear to install but it ended up working fantastic. No more hoping the antenna is turning and no more hoping it is aligned with what the controller says!
Mine spins slow on really cold days but with the feedback pot installed, I can actually see if it is turning or not.

I also found a control unit for an actual CDE controller that has a nice and large illuminated directional meter in it. I think it drove a motor unit that required a 5 wire cable so its feedback circuit worked. I don't recall the actual CDE models the controller worked with. I have two of them, one has a gold colored meter and is marked with amateur markings on it and the other simply has a silver and clear meter and was sold more for the TV users back in the day.

I was able to modify my controllers so they work with the cheap TV rotors still sold out there for under $100 dollars.

This thing is dead on with the direction it tells me I'm aiming. It has to be otherwise I have an issue with the feedback pot outdoors or my gear has broken!

I eventually mounted a weather cam above my Maxrad yagi and can also use that to see which way I'm aimed.
I ended up changing the aim of the WX cam though and it is about 90 degrees off from the true antenna aim.
I did this as my primary aim of the yagi put the WX cam aiming at the woods which was useless.

As far as reducing simulcast issues, I could not be happier!
It worked fantastic for that and was one of the original reasons I even put the yagi up. The main reason was to increase signal strength from the main system I used to monitor in the 800 MHz band.
Then I found it also came in very handy for determining a sites direction when working with multi site 800 MHz trunked systems such as Ameren Electric uses here in the Midwest.
It also worked very well for that experiment once you nail down the sites expected direction. Sometimes you think you are aiming at a site only to find out the signal is actually coming in from the back side of the Maxrad yagi.
A solid panel corner reflector helped me with that problem and also did wonders in reducing or eliminating simulcast issues. The corner reflector did such a good job that I quickly found that it was wiping out signals that the yagi would still pickup from the side. The corner reflector knocked these signals too low so the 12 element yagi went back up.

Anyway, a yagi can indeed help in reducing or even eliminating simulcast or LSM problems as long as your sites towers are far enough apart that the directional properties of your yagi can work with. Yagi's with a wide beamwidth will not help as well as a yagi with a narrower beamwidth for example. All these specs should be available with or from the manufacturer. If they don't offer these specs, you are probably better off looking for a different brand as they are probably Chinese knock offs that have never seen the light of an antenna signal pattern analyzer.
 
Status
Not open for further replies.
Top