Antenna Help!!

[emtunderwood]

Ok, I have a question or concerns. I have gathered some information and I'm posting this too the masses for some help and clarification on a conversation that I had with Sinclair and Comprod in the past few days.

First, I know I've posted about repeater problems and what not. It has came to conclusion that we have antenna and coax issues, as well as, desense issue, not seen at the ground level but it is happening at the antenna level.

Commscope makes our ava5-50 and they wanted me to check the ohn resistance at the ground level. I followed the steps for the ohm probes the outter ground of the hardline was zero and center pin was 19ohm with a mild wind and upto almost 40 in stronger winds. Commscope believes there is a bad connector at the antenna or on on the jumper to hardline. So, ok gotta get that fixed too.

However, speaking with Comprod and discussing antenna options they are suggestion a 884-70 in bi-directional pattern or 872-70 facing our main coverage area and leaving everything behind the antenna as unity gain. The concurred with commscope as there is definitely something concerning with the ohm loss and increasing impedance with increased winds.

Speaking with sinclair and discussing our problems and potential solutions. 1. move our antenna from 3ft off the tower to 6ft. and dropping our antenna from 305ft which is 567ft. AGL down to 190ft. and 460ft AGL or farther but woulding drop the AGL below 400ft. He stated that VHF likes to run along terrain instead of where it is now up in the air with no obstructions and according to his calculations the beam isn't coming close to terrain for 33mile away completely overshooting our area. He stated that the Commander 220-3BN at 8.1dbi where ok if we were in flat land and trying to talk out to 60mile mark or farther. He stated that the dbi needed to be about 5-6 and since our ERP is rated at 180watt and 280watt EIRP that we need to saturate our area and would bring the coverage area in closer to 20-25mile mark. He was explaining that down-tilt was not neccessary in this situation due to terrain. He suggestted SC281-HF3LDF and SC251HF3LDFD. He suggested this be placed 6ft from the tower, with the area with least coverage behind the antenna because it is going to null out. Sinclair also stated that we maybe trying to push too much over a mountainous area and needed 2 sites.

A local radio tower/communications company is telling us this is all BS that we need to install a DB-224 at the current height and we will talk better than we ever have and they only require a 13in stand-off. Well, I'm not sold on that with as many as they have to replace here in mountainous WV and the ones they have installed have worked worse than what they have replaced.

So what is y'alls take on this and thoughts on a solution.

Thank You

 

[prcguy]

Looks like you are measuring resistance across your antenna connector and while I agree it looks like a problem when the wind is blowing, you are not measuring impedance, you are measuring resistance. The antenna is apparently grounded and your ohmmeter is measuring the round trip from the center conductor up the coax to the grounded antenna, then back down to the shield side of the connector showing a high resistance connection where it would probably be just a few ohms if all connections were good.

As for antennas vs height vs spacing from the tower, I would not lower the antenna because in the VHF band with 5 to 6dBd gain antennas and only 567ft AGL, you should not have any overshoot problems and you should not need down tilt. For best omni coverage I would definitely move the antenna out away from the tower as far as possible and 6ft at VHF is a good start. The Commander 220 is a good omni antenna for low sites like yours but only you can decide if a bi-directional pattern is best for your type use.

 

[emtunderwood]

What about exposed dipoles?

 

[prcguy]

I like exposed dipoles, they have the most gain per linear foot over collinear types and they are much better on tall mountain tops where you get severe weather and icing.

What about exposed dipoles?

 

[Ubbe]

If you have desense issues you have very strong RF at your location. Connecting a ohm meter to an antenna with high RF levels coming out from it will make the readings false. When the wind blows the antenna probably bends and receives worse and the RF level drops and the readings change.

Compare it to trying to make resistance measurements in a circuit that are still powered on.

/Ubbe

 

[prcguy]

Repeater desense is almost always a mis-tuned or inadequate duplexer. It would take quite a bit of RF at VHF to make an ohmmeter across the antenna connector change readings, but it does happen.

If you have desense issues you have very strong RF at your location. Connecting a ohm meter to an antenna with high RF levels coming out from it will make the readings false. When the wind blows the antenna probably bends and receives worse and the RF level drops and the readings change.

Compare it to trying to make resistance measurements in a circuit that are still powered on.

/Ubbe

 

[W5lz]

If it's resistance changes with movement, then you have a problem with either the feed line or the antenna. There will be no change in resistance if things are installed correctly. I can see where knowing the resistance of the antenna system could come in handy, but in general it's pretty useless.
Desensing always occurs at the receiver. The cause of that desensing can be at the antenna end or the feed line. It doesn't necessarily mean it's a problem with your system, it could be the result of whatever else is on that tower. Unless you own that tower changing someone else's equipment is like spitting into the wind... forget it,

 

[paulears]

I'm always surprised that installers don't measure the resistance of the installation when it's commissioned and write out down somewhere. Brand new condition 8.5Ohms and a date. Then two years later when you have a problem, you can take another reading, and if it's still 8.5 Ohms you can make sensible decisions. A reading when there is a fault is meaningless without prior history.

 

[mmckenna]

I'm always surprised that installers don't measure the resistance of the installation when it's commissioned and write out down somewhere. Brand new condition 8.5Ohms and a date. Then two years later when you have a problem, you can take another reading, and if it's still 8.5 Ohms you can make sensible decisions. A reading when there is a fault is meaningless without prior history.

An installer worth the money they are being paid would sweep the cable/antenna as part of the install. The job wouldn't be complete unless it was working as designed.

Ohm readings won't do much good on non-grounded antennas, like a quarter wave. They'd show 0Ω if they were good, and would show 0Ω if the cable was open. "DC Grounded" antennas would show a dead short on a reasonable length of cable if good, and would show a dead short if it had a dead short in the coax or connector.

 

[paulears]

Take your point but a baseline would point out deterioration or faults - UR67 gives a reading of just under .5 Ohm for 50m, although on my repeater the TX feeder at about 30m reads just over 1 Ohm - which is higher than the cable specs say it should be. However, it's been that for nearly three years, with a Jaybeam commercial vertical on top. Suddenly getting 2 Ohm, or more would indicate probably water ingress I suspect here.

 

[Ubbe]

I'm always surprised that installers don't measure the resistance of the installation when it's commissioned and write out down somewhere.

Not the resistance but everything else. I've installed some antennas and also when subcontractors do the installation it is always sweeped by a SiteMaster antenna analyzer and stored and printed for future reference. I also do a spectrum sweep to get the signal levels from all other sites in the same frequency band as a reference to compare with at the next annual check. If you don't do the documentation then it's much harder detecting any problems with the installation and also the customer usually require full documentation including photos of the install with guy wires and everything for insurance and other purposes.

/Ubbe

 

[emtunderwood]

I think the department is going to replace both antennas and feedlines. There's nothing else left that hasn't been replaced. So, now poses a question, Omni stick or exposed dipole? Sinclair still recommending SC251-HF3LDF, SC281-HF3LDF and for Dipole SD2352-SF2PALDF and mounting whichever one 7.5ft from tower based on Sinclair recommendations. Comprod still recommending 884-70.

What are your thoughts, pros/cons and why?

Thanks,

Chief 801

 

[WA0CBW]

Why not sweep the lines first? You might find something that can easily be corrected.
bb

 

[emtunderwood]

Why not sweep the lines first? You might find something that can easily be corrected.
bb

The lines have been swept and being swept again by another group of techs on friday, but with the description of the problems we are having, both techs are saying theres something broke either in the antenna or a connector high on the tower. Still we are getting a 1.1-1.2:1 VSWR and .5 to 1w reflected on the ground. There's alot of popping and cracking and variance in coverage (talked in this area a hour ago, now it's dead but goto a known area of being horrible and guess what it works) it constantly changes. Both the antenna manufacturers and hardline manufacturer all 3 saying antenna and coax issues, and that a sweep can still show clean when actually its not) I mean this is frustrating as it can get.

 

[prcguy]

Are the techs moving the coax around at both ends while taking measurements to look for intermittent connections? At this point it sounds like the crackling noise might be caused by the antenna flexing and temporarily swapping to another antenna can rule that out.

Crackling and poor repeater performance can also be in the duplexer, they can get corroded connections inside and cause exactly what you are describing. Somebody needs to spend some time with a load right on the duplexer and a couple of radios nearby to see if the crackling shows up without the antenna connected. Move all cables and bang on the duplexer while doing this to try and force the problem to show itself.

Right now the problem could be anywhere from the receiver to the cables in the duplexer, the duplexer itself, the feedline and connectors and the antenna and its connectors. Sometimes you have to break the chain somewhere to rule out major parts of the system until it finally gets narrowed down to a specific item.

The lines have been swept and being swept again by another group of techs on friday, but with the description of the problems we are having, both techs are saying theres something broke either in the antenna or a connector high on the tower. Still we are getting a 1.1-1.2:1 VSWR and .5 to 1w reflected on the ground. There's alot of popping and cracking and variance in coverage (talked in this area a hour ago, now it's dead but goto a known area of being horrible and guess what it works) it constantly changes. Both the antenna manufacturers and hardline manufacturer all 3 saying antenna and coax issues, and that a sweep can still show clean when actually its not) I mean this is frustrating as it can get.

 

[mmckenna]

Went through something similar a few months ago.
Bad connector on the feedline at the antenna end.

Rather than spending a bunch of taxpayer money on new antennas and feedline, send someone up the tower to start looking at individual components. Blindly replacing stuff is an expensive approach. You've troubleshot it this far, don't give up.

 

[WA0CBW]

Sorry, didn't read back far enough to see you had already swept the lines. As prcguy said you need to break the chain somewhere. Maybe the solution was determined before the problem has been found.
bb

 

[prcguy]

Where are you located? If within easy driving distance I can locate the problem and for free.

The lines have been swept and being swept again by another group of techs on friday, but with the description of the problems we are having, both techs are saying theres something broke either in the antenna or a connector high on the tower. Still we are getting a 1.1-1.2:1 VSWR and .5 to 1w reflected on the ground. There's alot of popping and cracking and variance in coverage (talked in this area a hour ago, now it's dead but goto a known area of being horrible and guess what it works) it constantly changes. Both the antenna manufacturers and hardline manufacturer all 3 saying antenna and coax issues, and that a sweep can still show clean when actually its not) I mean this is frustrating as it can get.

 

[emtunderwood]

Here are the sweeps

 

[prcguy]

Am I reading that right? You have over 400ft of feedline?

Here are the sweeps