GTR8000 Pre-selector question.

[emtunderwood]

I'd agree, but the repeater hears the subscribers just fine. He's got lots of elevation, but with out knowing the terrain where he is, maybe it's too much gain and the pattern is shooting RF over the coverage area. But with upwards of 500w ERP, you would think that 99sq miles of needed coverage would be saturated and a couple inches of wet noodle would be enough to hear the repeater.

I've never been a fan of side mounted antennas. I've witnessed the effects of tower shielding with sidemounted antennas. I volunteered with a department that had a 4 bay, 1/4 wave spaced dipole array side mounted with out any standoff, and the tower shielding was so bad, a 10 minute drive away, standing with a portable directly behind the antenna, with the tower shielding it, you could hardly hear the repeater.

We have never noticed any shielding since the antenna change to the dipole 8ft from the tower from a stick 3ft from the tower. With the stick it was evident there was desense and reflection. Just something up with it. If we had our own tower it wouldn't be so bad to add another antenna and try some different things. Jumping through hoops with Crown Castle is a pain, well a nightmare. But at the moment it's our only option. For the past few years it's worked better than the old system but there are other systems in the area that do better that are at a lower elevation at half the height, that seem to talk forever. But ever since we've been there it's been that looming cloud something just isn't right.

 

[prcguy]

I'd agree, but the repeater hears the subscribers just fine. He's got lots of elevation, but with out knowing the terrain where he is, maybe it's too much gain and the pattern is shooting RF over the coverage area. But with upwards of 500w ERP, you would think that 99sq miles of needed coverage would be saturated and a couple inches of wet noodle would be enough to hear the repeater.

I've never been a fan of side mounted antennas. I've witnessed the effects of tower shielding with sidemounted antennas. I volunteered with a department that had a 4 bay, 1/4 wave spaced dipole array side mounted with out any standoff, and the tower shielding was so bad, a 10 minute drive away, standing with a portable directly behind the antenna, with the tower shielding it, you could hardly hear the repeater.

I think that antenna is rated around 6dBd gain (beamwidth 14-16deg) and its not very high, it should not have any overshoot unless the tower is on a big mountain. Most major antenna mfrs I've talked to about downtilt recommend 4deg of downtilt when you reach 10dBd gain (7deg beamwidth) and 1,000ft HAAT.

 

[Tech21]

What steps did you use to tune the pre-selector to the rx frequency? The GTR process is only a few simple steps and tuning only takes 5 minutes if you're working slow.

If tapping it affects the tuning, check the locking nuts.

 

[ElroyJetson]

Wait, what? You tune GTR preselectors in 5 minutes? I've done a few of them and they take a LOT longer than that every time.
Are you tuning it for the inverted Batwings shape on the network analyzer? It took a while for me to learn the process. It's more art than science, at least at first.

 

[Tech21]

You're tuning for peaks and dips, only having to move one cable around and change a setting on the service monitor once. It doesn't take long to tune for peaks and dips.

 

[kayn1n32008]

I think that antenna is rated around 6dBd gain (beamwidth 14-16deg) and its not very high, it should not have any overshoot unless the tower is on a big mountain. Most major antenna mfrs I've talked to about downtilt recommend 4deg of downtilt when you reach 10dBd gain (7deg beamwidth) and 1,000ft HAAT.

I think it's 6dBd in omni pattern and ~8dBd in offset.

I agree, I'm thinking 8⁰ of down tilt is excessive, and if vertical pattern is that important, or the repeater is that much higher above the area being serviced, using less gain would likely be a better option than using down tilt.

Still, something isn't sitting right having the better part of 500w ERP and not being able to hear the repeater, but being able to get into it with 5w portables. There is definitely no issues with the receive chain, and adding a preselector will only hurt receive sensitivity at this point.

It's almost always the opposite, can't hit the repeater with 5w portables, but you can hear it 50 miles out.

 

[emtunderwood]

So. I did some math today. If given roughly 1600ft elevation at our antenna height to the horizon is 49mi. Given the -4degree downtilt antenna. and d=height/tan(-4) give a yield of 1381 subtract that from the height and your adjusted horizon would be at roughly 18mi. Now given that and shifting the beam width from 8deg above the horizon and 8 degree below at -3b above and below the horizon. That radiation pattern has now shifted and concentrated more within the 18mi and little past that point leaving 4degree and maybe -1.5db from 18mi out to 49mi. I have enclosed maps with zero downtilt and with 4 degree downtilt. Am I understanding this correctly?

 

[dickie757]

I use this for a rough idea

UISP Design Center

Your all-in-one suite for network planning and profitability tracking

ispdesign.ui.com

It is kind of intuitive. Mess around with the radios. It will mainly tell you LoS at your problem area. I dont sign in or keep track of anything on that site.

 

[ElroyJetson]

You're tuning for peaks and dips, only having to move one cable around and change a setting on the service monitor once. It doesn't take long to tune for peaks and dips.

I speculate you've never been trained on the Motorola training procedure for tuning the duplexers and preselectors that Motorola typically recommends with Quantars and GTRs. Just a hunch. If I'm right...you just don't know what you just don't know. Not trying to be offensive in any way, but it's so much more than just tuning for peaks and dips. Tuning a mobile duplexer to run with an Icom or Kenwood repeater being used by a school or taxi service barely resembles what's needed in an RF congested environment with repeaters operating in a mission critical public safety system. They're NOT the same. Not even close.

This is pretty much the same preselector tuning procedure. More to it than just a couple simple dips. And it's IMPORTANT to do it right, particularly in a digital environment. BER depends on it being right.

https://www.repeater-builder.com/motorola/mtr2k/pdf/mtr3000-preselector-tuning.pdf

 

[prcguy]

I speculate you've never been trained on the Motorola training procedure for tuning the duplexers and preselectors that Motorola typically recommends with Quantars and GTRs. Just a hunch. If I'm right...you just don't know what you just don't know. Not trying to be offensive in any way, but it's so much more than just tuning for peaks and dips. Tuning a mobile duplexer to run with an Icom or Kenwood repeater being used by a school or taxi service barely resembles what's needed in an RF congested environment with repeaters operating in a mission critical public safety system. They're NOT the same. Not even close.

This is pretty much the same preselector tuning procedure. More to it than just a couple simple dips. And it's IMPORTANT to do it right, particularly in a digital environment. BER depends on it being right.

https://www.repeater-builder.com/motorola/mtr2k/pdf/mtr3000-preselector-tuning.pdf

Ho hum, nothing special there. Its good instructions for someone with no experience tuning stuff up but for the experienced, just give us what you want the end result to look like and we will make it happen. Part of my job was tuning and aligning Klystron tubes up to 3.5kW that now cost $40k a pop and they have five tuning screws that can cost you $40k if you screw up bad enough. Or try aligning a satellite transponder group delay box sending a swept signal through an entire satellite uplink chain to the satellite then receive it to get amplitude ripple down to a few tenths of a dB and phase within a few degrees across 36MHz, I dare you to try it. After some training and a little experience with that stuff a GTR preselector is nothing to get worked up about.

 

[ElroyJetson]

That's true. Tuning cavity based devices can be quite the rabbit hole. One that goes very deep in some cases.

There are SO MANY RF applications that literally are impossible to implement without tuned cavity devices of specific types. They're a core technology, so much more than just a can with a screw jammed into it and RF connectors on the side of it.