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Author Topic: dBm vs. dB Loss  (Read 35554 times)

AF6D

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dBm vs. dB Loss
« on: March 30, 2016, 02:15:49 PM »

I am ashamed that I don'[t know the answer to this question. But I will ask anyway so that others might learn.

Our system uses a Sinclair Res-Loc duplexer. It has variable attenuation. I recall when the nOOb's installed it they noted -125dBm RX without the duplexer but -118dBm with the duplxer.

Does that mean that the duplexer is inserting 6dB loss? (due to being misaligned.) It shouldn't be more than 1.4dB.
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N8EKT

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RE: dBm vs. dB Loss
« Reply #1 on: March 30, 2016, 07:02:19 PM »

-118 is .282uv

-125 is .126uv

While .282uv sensitivity is quite good for any repeater, yes it is 7db of loss

A two-way shop that has a tracking generator and knows how to use it could likely improve on it though getting factory spec is difficult
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AF6D

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RE: dBm vs. dB Loss
« Reply #2 on: March 30, 2016, 09:32:08 PM »

Thank you for the reply. I noted that at a 6dB loss that is only 25% of the signal getting through. When the machine was at my home high in the mountains on an 85' tower using a DB224 it kicked butt! But now that its on a commercial site it doesn't do as well. Mind you that its a Motorola MSF 5000 on 2 meters. Hardly a POS. It is rated at -118dBm so we surmise that the receiver still works to spec.

The duplexer is a Res-Loc Q2330e that is supposed to only have 1.4dB loss on the RX side (I assume the TX side also.) The Res-Loc has adjustable attenuation by rotating the cavity but I cannot find tuning instructions. Do I just generate a signal and watch on the scope for minimum attenuation and turn the T-N connector/cavity accordingly? I mean, it doesn't have an I/O. Just a T connector. We own a HP 8924C and have the tools but not yet the wisdom!
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N8AUC

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RE: dBm vs. dB Loss
« Reply #3 on: March 31, 2016, 11:49:56 AM »

Well, it's a 6 cavity duplexer.
There should be 3 N connectors on the duplexer. One goes to the antenna, one goes to the transmitter, and one goes to the receiver.
In addition, there are cable connections between individual cavities.
A duplexer doesn't really have "variable attenuation".
Each cavity in the duplexer acts as a tuned circuit. You adjust the resonant frequency of each cavity.
When the resonant frequencies of each cavity are the same, this yields minimum insertion loss on the desired frequency, and maximum rejection of signals not on the desired frequency.
The three cavities on each side must be tuned to the same frequency.
You want the TX side to pass the TX frequency and reject all others, and the RX side to pass the RX frequency and reject all others.

You can find tuning instructions on repeater-builder.com
http://www.repeater-builder.com/antenna/sinclair/cm-1009.pdf

73 de N8AUC
Eric


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AF6D

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RE: dBm vs. dB Loss
« Reply #4 on: March 31, 2016, 12:34:51 PM »

Thank you for taking the time to reply. The Q2330e has variable attenuation that is achieved by loosening the locking screws and rotating. It is rated at 1.4dB loss - not 6-7dB that we have now. It is frequency tuned. I just located this but it isn't for the Q2330e - mislabled. http://www.repeater-builder.com/antenna/sinclair/q2330e-duplexer-tuning.pdf
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N8EKT

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RE: dBm vs. dB Loss
« Reply #5 on: March 31, 2016, 06:32:57 PM »

You really need a tracking generator to tune a duplexer well

The notches have to be wide enough to filter out all the sideband noise yet deep enough for adequate isolation

It's a tricky balance

If you simply tune for max pass and max notch depth (attenuation) sideband noise will cause desense and even the slightest drift due to temperature will again cause you desense

A shop with a tracking generator can literally tune a duplexer in just a few minutes so the charge for doing so is very minimal

If your HP8924 has a tracking generator then you can attempt it as described here:

http://www.sinctech.com/wp-content/uploads/2013/02/Q-Series-Res-Lok-Duplexers.pdf
« Last Edit: March 31, 2016, 06:40:02 PM by N8EKT »
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AF6D

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RE: dBm vs. dB Loss
« Reply #6 on: March 31, 2016, 10:33:55 PM »

Thank you for that document. We already have it but it too misses my question. By rotating the body of the cavity one may add insertion loss. In fact on one that we have it is marked with a Scripto. But the one at the site is not. The question isn't how to tune the Q2330e using our HP 8924C w/Tracking Generator and RLB, but how to tune the insertion loss. 6-7dB is far, far outside of the norm. There shouldn't be more than 1.5-3.0dB and this is a rather expensive and well regarded duplexer. We are at -125dBm with the duplexer and at -118dBm as the specifications of the MSF state. I nrealize that this may be a subtle point for most of you but make the difference to us. We can rotate the body of each cavity but how do we measure its loss?
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W9IQ

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RE: dBm vs. dB Loss
« Reply #7 on: April 02, 2016, 05:44:16 AM »

Bob,

I think you are misunderstanding the duplexer terminology. There is no "variable loss" feature per se in a duplexer. There are basically two adjustments on each duplexer can. One sets the pass frequency - that is the frequency that will pass through the can with minimum loss. This is adjusted with the large tuning rods on the cans (not rotating the cans as you suggest). The second adjustment is the reject adjustment - this is the frequency that the can will attenuate. This frequency is adjusted with the capacitor adjustment on each can.

There are therefore two losses that be can measured in the can. One is the loss that the pass frequency experiences. This should be less than about 0.5 dB per can. The other is the loss of the reject frequency. This should be very high in each can - in the order of 25 dB. The adjustment procedure described in the Sinclair document on repeater builder accurately describes how to adjust the duplexer to these specifications. If you have 6-7 dB path loss, then either you have a defective duplexer, the duplexer is not made for your frequency range, your test bench is not set-up correctly, or you are not following the proper procedures.

Your comment that you are at -125 dBm with the duplexer but -118 dBm without it is very confusing. The duplexer will lower the sensitivity of the receiver so if you start at -118 dBm bare receiver sensitivity, the receiver can detect a signal of 0.282 uV. The duplexer, when properly tuned, will cause the receive frequency to be attenuated by roughly 1.5 dB. This means your receiver system sensitivity will drop to about .335 uV or -116.5 dBm. When sensitivity goes down, the dBm number becomes greater (less negative).

What is also important is that the duplexer should reduce your transmitter power that gets into your receiver input by 85 dB or more. This is essential to minimize the desense from the transmitter and of course, in an extreme case, avoid blowing out the front end of the receiver. This is the closest you can come to the notion of desired attenuation from a duplexer - but this is not called "variable attenuation" in the trade. The correct term would be "reject loss" or "reject attenuation".

Your transmitter output power will suffer the same 1.5 dB loss as your receiver sensitivity does. So if your bare transmitter in your MSF-5000 puts out 75 watts, you will only get 53 watts out from your duplexer.

Also be aware that termination impedance has a profound effect on the performance of a duplexer. If you bench test a duplexer with laboratory grade loads and then connect it to an antenna system that not close to a 1:1 SWR, you may experience very different results in the field from what you measured on the bench. If you cannot bring down the SWR of the antenna, consider inserting a matching circuit between the duplexer and the antenna feedline to provide a better match.

Also don't overlook the need to use high quality, double shielded coax from your repeater connections to the duplexer and from the duplexer to the antenna. This applies on the test bench and in the field. Leakage from low quality coax can negate a substantial part of the isolation provided by a high quality duplexer and can throw off test procedure results.

I hope that helps your understanding.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N8EKT

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RE: dBm vs. dB Loss
« Reply #8 on: April 02, 2016, 06:28:07 AM »

"What is also important is that the duplexer should reduce your transmitter power that gets into your receiver input by 85 dB or more. This is essential to minimize the desense from the transmitter and of course, in an extreme case, avoid blowing out the front end of the receiver. This is the closest you can come to the notion of desired attenuation from a duplexer - but this is not called "variable attenuation" in the trade. The correct term would be "reject loss" or "reject attenuation"."

Glen is spot on and well said









« Last Edit: April 02, 2016, 06:31:41 AM by N8EKT »
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AF6D

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RE: dBm vs. dB Loss
« Reply #9 on: April 02, 2016, 12:17:35 PM »

Then I have been BS'd all along with this specific duplexer that is fed with RG214 and gold-plated connectors. Each cavity is rotatable and one (we own 2) has been marked and scored with the amount of loss desired. The BS is that rotating the cavities doesn't affect tuning that would account for a 6-7dB loss. It is spec'd as you noted as 1.4dB loss per side and was tuned by a communications tech from a well known company. When he left it was left to me with no instructions.
« Last Edit: April 02, 2016, 12:21:36 PM by AF6D »
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W9IQ

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RE: dBm vs. dB Loss
« Reply #10 on: April 02, 2016, 06:59:25 PM »

Bob,

I think you are still mixing up terminology. The 2330e duplexer consists of square cans. So you cannot rotate them. I believe you think that the threaded rods "rotate the can". This is not the case. The threaded rod adjusts the pass frequency.

It is possible that a tech, while tuning the pass frequency rods, noted the loss at the pass frequency  for various positions. But per my earlier post, to call this variable attenuation is completely wrong terminology.

Forget the markings and drop the wrong terminology - just tune the duplexers per the instructions. They are good duplexers.

- Glenn W9IQ
« Last Edit: April 02, 2016, 07:03:17 PM by W9IQ »
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

N8EKT

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RE: dBm vs. dB Loss
« Reply #11 on: April 02, 2016, 07:58:11 PM »

Here is a video from Sinclair that seems to be pretty good


https://www.youtube.com/watch?feature=player_detailpage&v=U7QZzzB2LZU


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W9IQ

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RE: dBm vs. dB Loss
« Reply #12 on: April 03, 2016, 05:04:31 AM »

Nice video. Thanks for sharing the link.

- Glenn W9IQ
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- Glenn W9IQ

God runs electromagnetics on Monday, Wednesday and Friday by the wave theory and the devil runs it on Tuesday, Thursday and Saturday by the Quantum theory.

AF6D

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RE: dBm vs. dB Loss
« Reply #13 on: April 04, 2016, 05:42:26 PM »

Thanks Glen and for your patience. Respectfully, I am not confused as to what my Elmer told me.  I am new to it and was simply told that loosening the screws (not the rod) rotating the loops causes the insertion loss to change what cannot be changed. One is marked with a Scripto; one is not. I'd rather be embarrassed now than waste more time trying to find about 4.5dB of loss. I too have read the spec sheet but have never seen a tuning sheet for this particular model. Now the issue I have, with a red face but eager to learn, is solving the loss issue. I have never tuned a duplexer. My in-person Elmer was always too busy and just did it. "I'll show you later..." I have the 8924C and I'm learning.

The duplexer was tuned. I know that it was because the INVAR rods had been cut by a disreputable eBay seller to "allow shipping" although the duplexer was in the 170's. He said nothing until after the sale. Hard lessen. I had to source INVAR when Sinclair wanted $120 a rod. We bought feet of it for $35 and the cans tuned.

We also have a co-tenant with remote bases using untwisted Bell wiring that has never had a co-inhabitant and especially one with a Station Master near his top of tower spot taking out his GM300's and Alincos with a MSF 5000. He's pissed and screwing with us. On top of this we are regularly jammed with a low power carrier 12.5KHz off frequency. It is close to the repeater. He has a camera in the vault and we don't quite seem to locate the damn thing. It is remotely activated and someone kerchunks us to set it off. It has no PL but rides the hang timer until the TOT. I monitor the input from my mountain home 1 mile from the commercial site and listen to it happening. I suspect that the frequency agile remote base portion is being parked just off of our input to cause interference and discourage use of the machine. The MSF is 30KHz with a 2MHz receive window. Not a good choice in southern Calif. with 15KHz channel spacing. This is the bigger issue. When the machine was located at another site it truly kicked butt from 6,450 feet. I only mention these minor details to point out that I'm only half the idiot you may think. ;-)
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AF6D

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RE: dBm vs. dB Loss
« Reply #14 on: April 04, 2016, 06:07:12 PM »

Great video! I love the part about the tools. I simple screw driver. Sinclair insisted that in order to buy the INVAR rods we had to buy the complete loop/ We asked why. Just because! BUT - it comes with a "special" tuning screw driver. Like I said I'm only half the idiot assumed and sourced INVAR away from Tessco and Sinclair. Unfortunately this assumes that one has a network analyzer rather than a tracking spectrum analyzer. But I get that it will need to be a bit more work.
« Last Edit: April 04, 2016, 09:18:47 PM by AF6D »
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