DB224 refuses to duplex

[WRMD298]

Greetings,
I have a top mounted DB224 which doesn't want to duplex with any appreciable power anymore without receiver desense. Here are the specs:

I have tried four different repeaters - Motorola MTR2000 100-watt, MTR2000 40-watt, Quantar, and Kenwood TKR-750. The 100-watt version of the MTR2000 has been modified to work below 150 MHz but still has the original 150-174 circulator in the PA from the factory.

I have tried several different duplexers - brand new, Sinclair Q2220E with 75-ish dB of notch, Sinclair Q202 with 85-ish dB of notch, Telewave TPRD1556 with 115 dB of notch, and finally the beast, a Wacom WP-643 with so much reject notch that I can barely measure it on the tracking gen (bottoms out at 120 dB of notch).

The antenna is a 2014 model DB224. It is a 150-160 model and my frequency is 146.985. It was installed professionally 10 years ago with CommScope FXL-780 cable. There's 480 feet of cable, and 2.2 dB of calculated loss. The VSWR in the building at the transmit freq. is 1.5:1. Yes, I know that you "shouldn't use your duplexer as an antenna tuner" so I have done the best I could - tune the duplexer on the bench with 50 ohm equipment and then use an EMR corp Z-matcher to make the antenna look like it's 50 ohms in the building so the duplexer tuning doesn't shift.

all interconnect cables are 1/2" Superflex or 1/4" Heliax. No braided cable anywhere. The system duplexes perfectly fine when I attach a Bird Termaline load at the bulkhead, keeping the PolyPhaser and Z-matcher in circuit. No problems whatsoever.

I put the repeater on this antenna 6 years ago. I was able to run full power with no desense or duplex noise. Over the past year I have noticed the RX sensitivity slowly degrade. That's when I did the desense test, and sure enough, about 8 to 10 dB of desense with 100 W out of the transmitter. I started taming the power down, then changing repeaters, trying different duplexers, and anything else you can think of with no help. The amount of desense does go down when the TX power is reduced. To get ALL of the desense gone, I can only run 25 watts out of the repeater. The decrease in performance is felt in the middle of our towns in a mobile where the noise floor is high.

I have a spare antenna on the same tower, just down the tower a little ways standing off about 6 feet from the tower, fed with FXL-540 hard-line. The antenna is a Diamond X50. It duplexes perfectly fine with full power with NO desense (odd, because I would normally never trust a ham-grade dual band antenna for moderate-power duplex use).

There have been no changes in the site, and the antenna is high and in the clear. A tower crew did an inspection and everything up top is tight and clean.

I have always liked the DB224 antennas but I am starting to wonder if there is white chalky corrosion building up between the elements and mast that can cause broadband noise when it's "lit up" by RF. I know also the DB line of antennas had a round of bad phasing harnesses but I am thinking that a model as late as 2014 would have been in the clear of this. Again, the antenna sweeps fine and the match is right where it should be.

Any thoughts?

 

[KevinC]

I would suspect PIM. It will probably sweep good all day long, but once you apply power it will act up. Oxidation/corrosion in/on the antenna or phasing harness somewhere.

 

[WRMD298]

So the short answer is to (probably) replace the antenna.

There is something else that I forgot to mention.. The tower crew does TIA inspections every so often and they deemed it necessary to install a lightning rod out the top of the tower. The DB224 was the highest thing, being 22 feet tall. The new lightning rod they put up is about 10 feet away from the antenna, but also out the top of the tower on the other leg of the candelabra. The rod is a 20-foot piece of galvanized steel with a 6-foot copper ground rod bolted to it. I am going to guarantee there is zero no-oxide compound between the copper rod and the galvanized steel. Would this be a suspect for PIM? (I'm guessing yes). Unfortunately I don't know to the date when this was done so I can't directly blame this on my current problem. See attached pic. You can see the lightning rod in the background. Although it doesn't look like it, the tip is about 4 feet taller than the DB224. See anything else suspect up there? The lighting system is a Drake LED.

Have you or anyone else seen problems with PIM on late DB224's ??

 

[Project25_MASTR]

We only consider 224's to have a 5 year service life due to all the issues related with water ingress on the phasing harnesses. I won't buy CommScope antennas anymore these days...

 

[jeepsandradios]

I run the DB224 on multiple repeaters and have some almost 25+ years old still working. I'd have someone do a true sweep of the antenna system to start. I know the tower crew climbed and looked but there could be damage they did not see. Its out of band more than I would like but if its worked for years and now doesn't it may be a bad harness. I'm surprosed you got good vswr becasue none of mine will go down the ham bands and actuially sweep well or show decent VSWR.

 

[WRMD298]

I am the someone who does the sweeps (part of my daytime job). Anritsu 820 analyzer and all looks great. I am thinking PIM. Today I checked the machine and all seems OK at 30 watts out of the duplexer. As far as the VSWR, I'm getting 1.5:1 in the building at ~147 MHz. With 2.2 dB of line loss, that would mean the antenna actually has a return loss of 9.6 or right at 2:1 if you were to measure it AT the antenna. That's about what all of the DB224's in the 150-160 MHz range have that I've checked but yours may be different..

 

[WB5UOM]

I have had exactly ONE 'commscope' DB-224 issue when they first took it over, the harness, as mentioned was not heat sealed properly and it filled with water.
Tessco replaced it for me due to my volume I was using them for at the time. That replacement antenna is still at 460 ft playing like a song.

I have so many DB-224 in service its hard to count.
Interesting problem here, hopefully problem can be found AND posted here.

 

[kayn1n32008]

I'd bet the harness has had water ingress, and is pooched.

Replace it with a Sinclair or Comprod. You also won't have e to worry about SWR. An SRL210-C4 is a full VHF band antenna.

 

[prcguy]

I would temporarily replace the DB-224 and bring it down for testing on the ground. You can test under isolated conditions with known good coax and patch it to the repeater looking for desense. You can also sniff it with a Yagi and spectrum analyzer looking for IMD generation. If it has problems it may only be a loose connection or something else that can be addresses.

 

[WRMD298]

I agree with the procedure. I've duplexed antennas near ground level and wiggled the harness and elements around to search for noise. Unfortunately, it's up at 410 feet and I can't get it down easily or practically. Today it seems to be duplexing fine with the repeater running 40W and therefore getting 30W out of the duplexer leaving the building. The repeater is fairly balanced now with an average mobile. The wind was pretty calm today and the humidity was up. It seems like things act up when the air is very dry and the wind is blowing.

If I do get the chance to get it down, I'll have something else permanent to put up and replace it. I'm thinking a low PIM heavy duty fiberglass if I had the chance - one that doesn't flex in the wind like the DB Spectra 3 dBd model that's about 14 feet tall. I'm losing faith in antennas with exposed dipoles and external harness junctions. If I went back with a folded dipole, it would be a Sinclair or Comprod with internal harness and welded elements.

Being that I can afford neither, and there's not enough interest or use in the repeater to even consider taking up a collection to replace an antenna that's working, but not working as perfect as I would like to see it, we will probably just leave it alone for now and assume the desense is coming from PIM due to the antenna itself or something else up there that's corroding that I don't know about.

 

[prcguy]

I agree with the procedure. I've duplexed antennas near ground level and wiggled the harness and elements around to search for noise. Unfortunately, it's up at 410 feet and I can't get it down easily or practically. Today it seems to be duplexing fine with the repeater running 40W and therefore getting 30W out of the duplexer leaving the building. The repeater is fairly balanced now with an average mobile. The wind was pretty calm today and the humidity was up. It seems like things act up when the air is very dry and the wind is blowing.

If I do get the chance to get it down, I'll have something else permanent to put up and replace it. I'm thinking a low PIM heavy duty fiberglass if I had the chance - one that doesn't flex in the wind like the DB Spectra 3 dBd model that's about 14 feet tall. I'm losing faith in antennas with exposed dipoles and external harness junctions. If I went back with a folded dipole, it would be a Sinclair or Comprod with internal harness and welded elements.

Being that I can afford neither, and there's not enough interest or use in the repeater to even consider taking up a collection to replace an antenna that's working, but not working as perfect as I would like to see it, we will probably just leave it alone for now and assume the desense is coming from PIM due to the antenna itself or something else up there that's corroding that I don't know about.

It’s typically the opposite with people fed up with fiberglass antennas that have developed noise from bad internal connections caused by the antenna swaying in the wind or from corroded dissimilar metals. I saw this recently with a Sinclair VHF antenna on a local 2m repeater and a10dB SuperStationmaster I bought.

 

[buddrousa]

Reread the 1st post back to the point swap the antenna

 

[WRMD298]

Reread the 1st post back to the point swap the antenna

Yes, I have read the posts. All of them. Thanks. Not as easy as just saying "swap the antenna"

 

[buddrousa]

Well from an old tower climber I say this.
1. Time to pay a climber to go and do a few test. Put a Watt Meter between the Hardline and the Antenna. If that looks good replace the Antenna with a Dummy Load. You are going to find a bad antenna or bad hardline.
2. Use the Spare Antenna and Hardline you have on the tower now.

 

[WRMD298]

It’s typically the opposite with people fed up with fiberglass antennas that have developed noise from bad internal connections caused by the antenna swaying in the wind or from corroded dissimilar metals. I saw this recently with a Sinclair VHF antenna on a local 2m repeater and a10dB SuperStationmaster I bought.

10/4 point taken.

 

[WRMD298]

Well from an old tower climber I say this.
1. Time to pay a climber to go and do a few test. Put a Watt Meter between the Hardline and the Antenna. If that looks good replace the Antenna with a Dummy Load. You are going to find a bad antenna or bad hardline.
2. Use the Spare Antenna and Hardline you have on the tower now.

10/4 thanks.

 

[lenk911]

Replace it with a Sinclair or Comprod. You also won't have e to worry about SWR. An SRL210-C4 is a full VHF band antenna.

While these are great antennas they are a little different than the DB-224/264 series. None are plotted as true omni. When you order, you order them with field adjustable dipole spacing or fixed spacing. I never use the adjustable because of wind loading and something else to go wrong. The permanent spacing antennas come in 1/4, 3/8 and 1/2 wave dipole spacing. Price is the same but if you leave off the spacing from the order you may get an unpleasant surprise! Order as "-025, -0375 or -0500" The wider the spacing the closer to omni you get. The other beauty is they resonate from 132-174 MHZ.

They also have a non catalog special of 2 dipoles on VHF and 2 or 4 on UHF with separate feed lines on a 20' stick. Great for emergency management where you can have one 132-174 station and one 440-470 MHZ station on the same antenna! Use a cross band coupler and combine both on one line! Great for counties that have an active ham group as part of the EM team. The tower gets a professional grade antenna professionally installed for the hams!

The downside of the 3/8 and 1/2 wave is they place a larger bending moment on the mounts. If these are top mounted on a tower, it wont be long before they are leaning. If side mounted, I specify a fiberglass tip stabilizer on them to relieve the stress.

 

[KevinC]

Yes, I have read the posts. All of them. Thanks. Not as easy as just saying "swap the antenna"

When I first encountered PIM it wasn't a "thing". It was a tough call getting a tower climb because that the large ESMR I worked for at the time had never heard of a "noisy" antenna. They finally agreed and we swapped the antenna. It was a panel antenna and once I took it apart I found the corrosion on some of the joints. After that we had a lot of this same antenna model do this and it was just standard procedure to swap them. Being on the Gulf coast is hard on antennas.

In case anyone is curious it was the Antel RWA-80017.

 

[WRMD298]

While these are great antennas they are a little different than the DB-224/264 series. None are plotted as true omni. When you order, you order them with field adjustable dipole spacing or fixed spacing. I never use the adjustable because of wind loading and something else to go wrong. The permanent spacing antennas come in 1/4, 3/8 and 1/2 wave dipole spacing. Price is the same but if you leave off the spacing from the order you may get an unpleasant surprise! Order as "-025, -0375 or -0500" The wider the spacing the closer to omni you get. The other beauty is they resonate from 132-174 MHZ.

They also have a non catalog special of 2 dipoles on VHF and 2 or 4 on UHF with separate feed lines on a 20' stick. Great for emergency management where you can have one 132-174 station and one 440-470 MHZ station on the same antenna! Use a cross band coupler and combine both on one line! Great for counties that have an active ham group as part of the EM team. The tower gets a professional grade antenna professionally installed for the hams!

The downside of the 3/8 and 1/2 wave is they place a larger bending moment on the mounts. If these are top mounted on a tower, it wont be long before they are leaning. If side mounted, I specify a fiberglass tip stabilizer on them to relieve the stress.

Yes sir, and thanks. I'm very familiar with the Comprod models and I really like the mechanical structure and the wide bandwidth. One of the emergency mgt. agencies close by has the very antenna you mentioned above with the VHF and UHF dipoles on the same mast. They do sweep wonderfully and have such a very wide band-width. One thing I noticed, however, on a dual 4-bay UHF array that was taken down from only 10 years' worth of service is the two RG-8 type coax leads coming from the bottom of the antenna were rotten and flaking away. I can't remember the brand name of the cable. Belden maybe. Whatever it was, the jacket was a porus material and it obiously took a toll due to UV exposure. We had to cut the bottom foot off the antenna just to be able to reach "clean" coax and put new connectors on it before we could use it. I will have to say the Decibel "Vapor-Bloc" cables are the BEST I know of. I have DB224's with 1977 date codes on them that were pulled out of service and the cable is still in great shape!

Also, understandable about the 1/4, 3/8 and 1/2 wave spacing. Most of the Comprods in my area are 1/4 wave spacing for offset use. Like you mentioned, I would be hesitant to mount one of these 22-foot antennas out the top of a tower unless I had a zero-torsion mount and then I would cringe every time we had a storm. That's a lot of antenna to be top mounted!

 

[WRMD298]

When I first encountered PIM it wasn't a "thing". It was a tough call getting a tower climb because that the large ESMR I worked for at the time had never heard of a "noisy" antenna. They finally agreed and we swapped the antenna. It was a panel antenna and once I took it apart I found the corrosion on some of the joints. After that we had a lot of this same antenna model do this and it was just standard procedure to swap them. Being on the Gulf coast is hard on antennas.

In case anyone is curious it was the Antel RWA-80017.

Yes, that's what I was thinking earlier. Checking the antenna with a watt meter or sweeping it with a VNA won't show PIM. You have to "light it up" before that can be detected.