Microwave Band Choice Backhaul TRS

[rr60]

Can you help me understand some of the reasons 11GHZ might be chosen for backhauls of a P25 TRS.

The reading I have done so far leads me to conclude that 11GHZ and up can suffer from rain fade. That is exactly what I am seeing, especially with thunderstorm cell penetration along the path.

Why choose 11 over some of the lower bands? TIA!

 

[GTR8000]

6 GHz is good for long hops with decent bandwidth/capacity; 11 GHz offers more bandwidth/capacity, but should only be used in short hops (under 10 miles).

 

[GTR8000]

Sorry, didn't really answer your whole question. Basically you would choose 11 GHz if you needed higher capacity than 6 GHz could offer. One real-world example might be the link between a primary core and a geo-redundant core, where the sites are just a few miles apart, and so the effects of rain fade would be negligible.

What system are you "seeing" this on? Are you looking at the actual microwave levels, or some other way of observing?

 

[prcguy]

The engineer designing the system should have calculated everything out including your rain region to provide a certain percentage of reliability like 99.97%. I would want to look at the design records then make a few measurements to see if your getting the expected receive level during clear sky, etc. If not the antennas may not be aligned, chipping away at your headroom and I would also see if someone has added more carriers than the original design called for, that would potentially reduce power in all other carriers. Finally, take a look at the license to see what power level or ERP is allowed and see if there is any system headroom. Sometimes you can bump the power up some to get over some rain fade.

I'm more of a satellite engineer but the same stuff applies for terrestrial and more like tropo ducting and phase cancellation over distance.

 

[rr60]

Sorry, didn't really answer your whole question. Basically you would choose 11 GHz if you needed higher capacity than 6 GHz could offer. One real-world example might be the link between a primary core and a geo-redundant core, where the sites are just a few miles apart, and so the effects of rain fade would be negligible.

What system are you "seeing" this on? Are you looking at the actual microwave levels, or some other way of observing?

Sub here. The system is Somerset County TRS. Very little if any information flows downhill. Trying to understand so as to be better prepared.

The license, path and equipment can be seen here. The path from Johnston Drive to Miller Lane is the one of interest.

https://wireless2.fcc.gov/UlsEntry/...tachmentKey=19063885&attachmentInd=applAttach

Suspected path fade is seen a few times a year only with very intense rain and even heavy snow. The path is 11.466 miles and does traverse significant terrain.

These suspect MW events coupled with TV 19 Tropo on Site (3) are making comm’s less than ideal.

No measuring equipment. Only thing that happens is the CC disappears during severe weather a few times a year.

Site Trunking mode does not operate either.

I am getting a better understanding, thanks to all for comments on this thread.

WQPN327

 

[kf8yk]

Sub here. The system is Somerset County TRS. Very little if any information flows downhill. Trying to understand so as to be better prepared.

Site Trunking mode does not operate either.

In a simulcast system isolated subsites typically do not enter site trunking. Having one subsite in site trunking and the remainder in wide trunking is a recipe for widespread interference. Site trunking should only activate when the prime subsite loses connectivity to the zone controller(s) and all the subsites enter site trunking together. Standard behavior is isolated subsites simply stop transmitting.

In addition to the previous comments other reasons for picking 11 GHz over 6 GHz: In your market the microwave bands are very congested, getting 6 GHz hops coordinated is more difficult. Another consideration is structural loading of larger 6 GHz dishes due to both the lower frequency and the need for sharper beam patterns for interference control in a crowded band.

 

[NVAGVUP]

Per comments above. Many reasons to use 11 GHZ vs 6 GHZ. Another reason not mentioned (Looking at your license), is if 6 GHZ were used/available, your system spare cards/modules would be increased. (Causing higher project costs). I suspect 11.44 m is at the threshold of 5 9 reliability. (The path needs to be run in a link planner to verify). Additionally, Exalt eq is no longer mfg. So even if 6 GHZ were available to license, the path would need a different mfg of eq.

Sometimes dish size comes into play. 6 GHZ requires a 6' dish minimum. (In almost all cases). 11 GHZ (per FCC and path planning) will permit dishes less than 6'. Maybe the towers will not pass structural analysis w/6' dish? (Per above)

Maybe the path frenel zone will not pass w/6 GHZ and pass w/11 GHZ?

In your climate and a 11.44 m hop, several events of rain/snow fade in a year does not surprise me.

 

[rr60]

Really great comments. Thanks all for participating!

 

[Cameron314]

That path is 7 miles with 4 foot dishes on each side. Expected RSSI is -32.6 which is pretty hot. I would guess it isn't that or you likely won't see drops. Maybe out of alignment a bit.

 

[NVAGVUP]

Per comments above. Many reasons to use 11 GHZ vs 6 GHZ. Another reason not mentioned (Looking at your license), is if 6 GHZ were used/available, your system spare cards/modules would be increased. (Causing higher project costs). I suspect 11.44 m is at the threshold of 5 9 reliability. (The path needs to be run in a link planner to verify). Additionally, Exalt eq is no longer mfg. So even if 6 GHZ were available to license, the path would need a different mfg of eq.

Sometimes dish size comes into play. 6 GHZ requires a 6' dish minimum. (In almost all cases). 11 GHZ (per FCC and path planning) will permit dishes less than 6'. Maybe the towers will not pass structural analysis w/6' dish? (Per above)

Maybe the path frenel zone will not pass w/6 GHZ and pass w/11 GHZ?

In your climate and a 11.44 m hop, several events of rain/snow fade in a year does not surprise me.

Edit- Kilometers vs Miles. (I need a refresh on metric system!) But same principals still apply.

 

[rr60]

That path is 7 miles with 4 foot dishes on each side. Expected RSSI is -32.6 which is pretty hot. I would guess it isn't that or you likely won't see drops. Maybe out of alignment a bit.

Thanks

 

[12dbsinad]

Pay close attention to the installation of the dish. There can't be room for error here, many times I've seen dishes move way to much because the structure is not rigid enough, or the tower guys just simply don't align it, or it's missing the return brace (see that often). Just be aware...

 

[rr60]

Thanks again everyone for outstanding comments. Now this dog will hunt🤣.

 

[rr60]

Alignment adjustments, so I am told. Thanks.

 

[crazyboy]

Alignment adjustments, so I am told. Thanks.

It will probably move again with that install.

 

[techman210]

That tower is going to gyrate more than Elvis’ hips in the wind unless a anti-torsion assembly has been attached.

if I’m reading the PCN correctly, it’s installed at 150’?

But looking at the picture again, there seems to be another structure behind that Rohn 25 type tower section that the stiffarm is attached to, which isn’t in the picture, so it may be good after all.

 

[prcguy]

That's not a Rohn 25, it looks like a large tower and the bottom section of the tower has the tower pictured as one of the three legs. Just above the dish the tower transitions to a single pipe for each of the three legs. In this case the tower section (leg) that the dish is mounted to will not rotate. Maybe the OP can verify this.

That tower is going to gyrate more than Elvis’ hips in the wind unless a anti-torsion assembly has been attached.

if I’m reading the PCN correctly, it’s installed at 150’?

But looking at the picture again, there seems to be another structure behind that Rohn 25 type tower section that the stiffarm is attached to, which isn’t in the picture, so it may be good after all.

 

[rr60]

Yes it is not Rohn 25. It is a 185’ tower. The photo is misleading. That is one of the three legs at that level.

 

[NVAGVUP]

If you have to pay someone to climb and realign dish, I would have them install a torsion/stabilization arm on the dish also. The pipe mount has minimal contact with tower, (Due to small diameter). It makes it hard to lock it down solid enough to withstand winds which can tweak alignment.

 

[rr60]

If you have to pay someone to climb and realign dish, I would have them install a torsion/stabilization arm on the dish also. The pipe mount has minimal contact with tower, (Due to small diameter). It makes it hard to lock it down solid enough to withstand winds which can tweak alignment.

Thanks for taking the time. Awesome thread.