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FCC Power Rules


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The thread here sparked an internal debate in my head. 47 CFR 95.1767(a)(1) states "The transmitter output power of mobile, repeater and base stations must not exceed 50 Watts." In the other thread, that version of the Bridgecom hardware provides 4dB of insertion loss before you even connect the antenna feed line, loosing 60% of your power right off the bat. In otherwords, before you eve get to feed line loss if you have 50W out of the repeater, you are down to 19W. Again, this further points to the importance of proper feed line choice and a great antenna (why skimp if you are already dropping $4k+ on a combiner). Even with hardline, you could only see 6.8W/10.7W/12.7W to the antenna with 1/2"/7/8"/1.25" on a halfway decent tower site (300'). 

Potentially, a manufacturer could potentially provide a 8U complete box with a single coaxial connector that puts out exactly 50W at that connector, but has all the combiner, transmitter & receiver in the single box, and if that enclosure package was tested and achieved the part 97 certification, would be acceptable for use for GMRS. The obvious would be that the transmitter would output higher than 50W to overcome the losses internal to the box, but since its "one piece" could potentially pass the transmitter based on that. This is slightly different from most repeaters, since the duplexer may have a spot in the case, the jumpers are exterior, and the rating is for the raw transmitter output before the duplexer. 

Now I have my opinion/interpretation, but I'd like to hear from others. Where is that power limit taken from and why?

This is not to say that anyone would know/notice/care if you ran an amplifier to make exactly 50W come out of the duplexer or combiner. Not saying you should, but its not impossible, not that there are part 95 certified amplifiers available that could handle the duty cycle needed.

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My read is that TPO is where the transmitting equipment meets the feed line. If transmitting equipment includes harmonic filter, circulator and a duplexer,, so be it, no problem. As long as the TPO is 50 watts.

The Motorola MSF5000 station integrated all of those optional components making a 100 watt station deliver 75 at the connector on the cabinet. This confused technicians who would see that the station was spec d at 100 watts without those internal options, and so they would turn the power up to squeeze 100 watts from the cabinet and that would make the PA run much harder.

Sent from my SM-T350 using Tapatalk

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2 hours ago, JLeikhim said:

This confused technicians who would see that the station was spec d at 100 watts without those internal options, and so they would turn the power up to squeeze 100 watts from the cabinet and that would make the PA run much harder.

I think I could still write a book on this subject. Or those that do this with every radio or related device they get their hands on. I still recall co-workers that tweaked "golden screwdriver" style their Maxtracs for as much power output as possible, then it was the Spectra's then Waris CDM series, and on and on, while they burned up radios. Some service shops have done this to create more paying work from their customers, though they will never admit to doing so. Most recently, I had this happen on the repair of several MTR2000 repeaters with failed power supplies. Repair/replace the power supply and find out they had tweaked output wattage to try to get max ouput power, which then bore down the power supply, shortening its life. Excellent point there. 

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As to legal power, I understand the rules to mean 50 watts (MAX) at the antenna port of the transmitter, as opposed to ERP (Effective Radiated Power/what the radiated signal is).

That said...

No matter what you do, there is going to be loss between the TX/RX and the antenna.  I don't know about 4dB 'in the box' but... 

The average (decent) duplexer has (or should have) 1.5dB or less insertion loss (each respective side, TX and RX), so there goes a 'watts' or so.  If you really like your TX, a circulator in the TX side is nice, but rarely done and, would likely add about, or just under, another dB of loss, so there goes another watt or so. 

It's fair to say that even if your TX is putting out 50W from the PA, by the time you connect to the antenna system, that 50W is going to be 30-40W (if you're lucky). 

That is, again, typical.  I'd add that the losses go both directions so your RX sensitivity just got a little worse as well (but not earth shattering).

This is why antenna system design is a make or break scenario.  The gain of the antenna ideally makes up for some of the line and perhaps even the duplexer loss and, if it has some gain, maybe focuses what energy it does radiate and receives where you want it.

As for tweaking the PA for more power, it doesn't really help, because once again, there is loss on the receiver side too, so upping the power is only going to make your repeater talk where it can't hear the people trying to talk into it.  As noted by another poster, it also risks cooking the PA itself and/or the power supply feeding it.

Now I know in the 'hobby' it isn't practical, but the ultimate goal of a repeater design/system is to be able to have the talk-in balanced with the talk-out for a given user radio and antenna combination.

You really can't do that thinking in watts, which is why the practice (for people who do all of the above) is to work in dB (decibels).  Why? Because losses and gains can be added up and subtracted to find out what you actually are dealing with.  Watts equates to a level of dB as does cable and other losses, antenna gain (or loss) and, the sensitivity of the receiver. 

Indeed, for all the hoopla about tweaking a PA and squeaking out a couple more watts, if you look at power output in dB as apposed to watts, you find even doubling the watts isn't really a whole lot of gain in the bigger picture.


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Hmmm, 4dB huh?  Yeah I suppose in a crappy duplexer.  Really shouldn't be that much loss in any decent duplexer.  Transmit combiner on the other hand can be more like 6 dB.  Which is what I have.  Of course I can connect 4 repeaters to that combiner and they can ALL transmit through it, and do.

Instead of worrying about the loss in you duplexer, and not having 50 watts at the port going up to the antenna, worry about installing an effective antenna system with sufficient height to provide the coverage that you desire for your repeater. 

My setup is as follows.

Receive antenna is at 220 foot at it's base, top of the tower.  DB408 currently and will be upgraded to a DB420 this spring.

Transmit antenna is at 110 feet.  Antenna is a Station Master.  It's side mounted and is shadowed to the South currently by the tower.  This will be replaces and relocated by a split DB420 (two DB408 antenna's on a common mast) that the top antenna will be on the ham combiner and the bottom will be connected to the lower antenna.  This will be relocated to the 180 foot level.

Point is, as it stands, on a clear day the system talks 60 miles in some directions.  And ONLY hears as well as it talks.  Meaning I am NOT transmitting into area's that I don't have the ability to hear a 35 watt mobile. 

I will always have some level of shadowing from the tower, due to the fact it's 20 feet wide at the 180 level.  But it's still able to talk a good distance in those area's that are not shadowed. 

There is some deeply rooted misconception that you need every last watt of power possible going to your antenna or your not able to talk.  And it's simply not true.  I have a commercial system over on the east side of Columbus that is 500 feet in the air.  The transmitters are set down to 20 watts because they will not turn down further.  It uses the same combiner I have with a 6 dB loss per port and 600 feet of feed line to the transmit antenna.  It can still be heard in Springfield, but has NO ability to hear that far for various reasons.  So it's talk out coverage is STILL with all that loss, far greater than it's ability to receive. 

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  • 1 month later...

Ok I'm new to this so please, If I'm just setting up a base station. why would I need a duplexer?  That's more for repeaters right?

So between the radio and antenna I have coax, a lightning arrestor. and ??? nothing. So if my radio puts out 50W (optimistic) and say 100 ft of coax. I know there are alot of variables here but why couldn't  I get estimated 40W to the antenna? 

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25 minutes ago, WRPQ991 said:

Ok I'm new to this so please, If I'm just setting up a base station. why would I need a duplexer?  That's more for repeaters right?

So between the radio and antenna I have coax, a lightning arrestor. and ??? nothing. So if my radio puts out 50W (optimistic) and say 100 ft of coax. I know there are alot of variables here but why couldn't  I get estimated 40W to the antenna? 

The reason a repeater runs a duplexer is the ability to receive and transmit simultaneously on the same antenna...to isolate the receiver from the transmit. You can absolutely run a repeater without a duplexer with separate Rx and tx antennas, and given adequate separation, will work fine.  That said, i do have a DIplexer in my shack, to allow 2m and 70cm monoband radios to feed one dual band antenna.

Power wise, the big variable is your coax. With low enough loss, 40 watts at the antenna isn't impossible. Case in point, I've done some testing with my vx4207. It shows 45 watts into a dummy load, or 43 into an antenna. With 17 or so ft of whatever coax Midland ships with it's bundles (it's unlabeled), I showed 23 watts at the antenna end. Swapped that for 35ft of abr400 (still all pl259/so239 connectors), and power at the antenna jumped to 35 watts.

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10 hours ago, WRPQ991 said:

So between the radio and antenna I have coax, a lightning arrestor. and ??? nothing. So if my radio puts out 50W (optimistic) and say 100 ft of coax. I know there are alot of variables here but why couldn't  I get estimated 40W to the antenna? 

Depends on the cable. 100' of LMR400 on UHF is about 3db of loss meaning a 50 radio will hit the antenna at 25 watts. RG58 is about 10db of loss so basically you'd have nothing at the antenna. 

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My interpretation is 50W max measured at the output of the transmitter (e.g. not at the duplexer).

That said, and as I've stated in another thread, start with forgetting about watts (for the moment), and recognize that 50W is roughly +47 dB.
Start subtracting all of the antenna system losses from that point.

Simple (conservative) example (insert or remove your own variables as desired)

Transmitter to duplexer jumper (6ft of RG142 @ 8 dB per 100ft) 0.54 dB of loss
Duplexer 0.8 to 1.5 dB of insertion loss. Lets use worse case 1.5 dB of loss
Duplexer to feed-line jumper (6ft of LMR400 @ 2.7dB per 100ft) 0.16 dB of loss  <Corrected!
1/2 inch hardline (50 ft @1.44 dB per 100ft) 0.72 dB of loss
Feed-line to antenna jumper(6ft of LMR400 @ 2.7 dB per 100ft) 0.16 dB of loss<Corrected!
Total antenna network loss (one way) 3.08 dB total antenna network loss loss <Corrected!
Power making it to the antenna = 43.9  dB or, 24.5 Watts (<Corrected!

Antenna Gain 6 dB (note not dBi) of "gain" ERP (Effective Radiated Power) of: 49.9 dB or  97.7W.<Corrected!

Also remember that the gains and losses go both directions!

Thanks To @Sshannonfor catching the error! :)

In most cases, jumpers between transmitter and receiver and duplexer are required but one need not necessarily use RG142 (Teflon) or 6ft of length.  Jumpers between the duplexer and feed-line (hard line/Heliax/whatever you call it) are commonly used as to connect "hard line" directly to a duplexer.  Same for the antenna to feed-line.  You don't strictly have to use them/do it as I have described, but it is good practice.

That said, substitute whatever numbers/components you like. In the case of the cables, use the loss specified as close to 465 Mhz as possible.

Finally, the point of this was to show that one starts by using the dB (not watts).  As good online calculator is: https://www.antennas.ca/calc_db_watts.htm

As seen in the above (conservative) example, one can vari the losses by a few tenths of a dB to several dB by the choices one makes.  In this example, the duplexer insertion loss could have been assumed to be the ideal 0.8 dB.  7/8" hardline could have been used instead of half inch, and of course the jumpers could be different cable and lengths.

All of the above assumes clean, properly installed, connectors, as a bad cable connector can really ruin ones day.

Hopefully this gives a better idea of how to think about antenna systems, repeater or base.

Lastly, looking at cable specifications, all cable is not created equal.  As popular as LMR400 seems to be with people, it's not all that great at UHF for runs over 50ft.  Hard Line/Heliax can be expensive, but if one is looking to get as much power to and from the antenna...

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1 hour ago, Sshannon said:

Is it possible that your calculations for losses for the 6 ft LMR400 are a tad high?

You know, I did miscalculate!

I pulled a "Michael Bolton" (office space) and put the decimal point in the wrong spot!  It's .167 dB for a 6ft jumper. a difference of 1.45 dB!  That changes the total loss to just 3.08 dB.  Therefore the power to the antenna is actually 43.9 dB or, 24.5W which with the antenna equates to (much better) ERP of 97.7 W.  Definitely an improvement.

Thanks for the catch!!


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  • 2 months later...

Thank you for the detailed analysis. This was very helpful as I go home tonight to fit a PA to my repeater. I am never one to push wattage as I understand the difference between 40 and 45 watts in db on the other end is usually negligible. I'm just bringing mine up to the point that transmit matches receive.

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