High Power vs. Low Power TX

billy7834 · Post by **billy7834** » Fri Aug 22, 2003 12:43 am

Was talking with friend who programs PD handhelds. He says low power transmit is used for repeater inputs and high power for simplex. I understand the high power for simplex for range., but why wouldn't you want high power for the repeater inputs? Wouldn't that allow you to hit the repeater from a further distance?

Nand · Post by **Nand** » Fri Aug 22, 2003 3:04 am

High power would increase the usable distance to a receiver. Perhaps they use mobile cross band repeaters with their portables and don’t need that much signal to reach the vehicle. Low power would be adequate in that case and reduce the battery power consumption while also preventing them from keying up other mobile repeaters that are more distant.

Nand.

billy7834 · Post by **billy7834** » Fri Aug 22, 2003 2:07 pm

Thanks for responding. No, they don't use mobile repeaters. The handhelds go direct to main repeater, same as the mobiles. This is in a large city.

nmfire10 · Post by **nmfire10** » Fri Aug 22, 2003 2:31 pm

If that is the case, then they must have some seriousl remote reciever coverage.

billy7834 · Post by **billy7834** » Fri Aug 22, 2003 4:09 pm

In any event, would there be any downside to using high power?

Post by **xmo** » Fri Aug 22, 2003 4:26 pm

"...would there be any downside to using high power?"
----------------------------------------------------------------

Besides reduced battery operating time and nuking your brain?

kurt meltzer · Post by **kurt meltzer** » Fri Aug 22, 2003 7:18 pm

On the police radio system I administer (UHF conventional with 22 voting receivers feeding spectra-tac comparators), I turned all the portables down from 4W to 2W. This has proven to be plenty adequate for in-building transmit, even though most of our personnel are using coiled cord speaker mics (antenna on radio) and radio is on belt during transmit. When we transmit from inside a building at 2W, we can always hit at least 1 or 2 of the nearest rx sites. Previously, when we would transmit at 4W, especially outdoors, we'd often unsquelch 6 or more sites. With more sites to choose from, the comparator had a higher probability of mis-voting a noisy site. So for me, it's been a voter performance issue. We do get longer portable battery life this way, but the primary reason I did it was to reduce wrong-site voting. I've never been fully satisfied with the performance of our Moto comparators, due in part to the decrepit telephone infrastructure in this area (suburban Chicago). Microwave is not an option for us due to its high cost and our inability to errect the towers necessary to achieve LOS paths (aesthetics). So, all my rcvrs are backhauled on conditioned 3002 circuits, but several of them fail to consistently exhibit their specified characteristics (uneven loss deviation between 2000-2500 hz among the various sites, which wreaks havoc with signal to noise voting) and getting telco to do anything about it has been all but impossible. So I've been forced to improvise this way, but it works good now.

RadioSouth · Post by **RadioSouth** » Fri Aug 22, 2003 7:29 pm

Sounds common in large cities. In NYC when the PD was using Sabers they were an SP model with a 1 watt output (repeat and T/A modes).

wa2zdy · Post by **wa2zdy** » Fri Aug 22, 2003 9:22 pm

Yep, no reason to run more power than necessary, and if trials showed low power to do the job, why waste the battery life?

billy7834 · Post by **billy7834** » Sat Aug 23, 2003 12:18 am

Thanks for the responses guys. Thinking in terms of a unit leaving city limits. What would the extra 2 watts be worth in miles from the closest repeater? (Approx) Thanks again.

Nand · Post by **Nand** » Sat Aug 23, 2003 5:34 am

Doubling the power from 2 to 4 Watts (a 3 dB increase) would also increase the received signal by 3 dB. If the received signal is 0.25 uV with a 2 Watt transmitter, it will be 0.35 uV when the TX power is increased to 4 Watt.

In free space or well in the clear, this would allow the transmitter to be moved 40 percent farther away from the receiver for the same received signal strength. On the ground with obstructions and reflections etc., this increase is more like a 10 percent increase in distance.

This is also the reason why changing a ¼ wave ground plain antenna to a 5/8 wave antenna does not always make a lot of sense in cities etc. It is only a very small increase in gain and the radiation pattern for the 5/8 wave is not as fat as the ¼ wave antenna.

Nand.

wa2zdy · Post by **wa2zdy** » Sat Aug 23, 2003 7:34 am

Nand is of course correct. The thing to remember about changing from a 1/4 to 5/8 (or on UHF even a collinear) is the gain works for your receiver as well as your transmitter. If that's an issue, it's worthy of consideration. Increasing transmitter power doesn't help you receive any better.

This of course is not practical with portable use. In the case of a high band portable with the standard helical "rubber duck," that antenna is worth about a 10 dB LOSS! So if your radio is putting out 5 watts, your ERP is closer to half a watt, and the receive loss is equal. (This is from studies done by GE in the 70s.) At least on UHF and 800, a 1/4 wave whip is practical even on a portable.

KB9WZD · Post by **KB9WZD** » Sat Aug 23, 2003 4:17 pm

This of course is not practical with portable use. In the case of a high band portable with the standard helical "rubber duck," that antenna is worth about a 10 dB LOSS! So if your radio is putting out 5 watts, your ERP is closer to half a watt, and the receive loss is equal. (This is from studies done by GE in the 70s.) At least on UHF and 800, a 1/4 wave whip is practical even on a portable.

Are you suggesting that my 5/8 wave telescopic antenna on my Bendix King is a little large for portable use

If so I agree with you but if I want to use it with our Amateur repeater I need the gain its that bad

wa2zdy · Post by **wa2zdy** » Sun Aug 24, 2003 3:41 am

Is walking around with an HT up to your lips with a 49 inch antenna sticking up practical? Since you don't say, perhaps you're on UHF, and even that . . . it's funny looking at least.

So lemme see here . . . a 5/8 on 2m/highband is about 49 inches. On 440 it's about 15 inches. You decide if the antenna connector on top of your radio can handle the torque of something that big flexing around in the air. If so, go for it. If not, hey, it's your radio to break. I know the techs on this board are always ready to help fix broken radios. Of course it's how they earn their living, so it won't be free, but . . .

Being completely serious though, if you need that much antenna to hit the "local" repeater, you need to reconsider your use of a handheld. Some things just aren't fated to be. As a younger ham, there was only one repeater in HT range - and it was as closed as the local bank on a Sunday. Thus even though I wanted to walk around town and be able to pull my trusty hand held out to chat, it wasn't fated to be. And it never did happen. (This was years before cell phones. I did actually have enough immediately local ham friends that I kept my radio with me on .52 simplex and it worked out ok, but I always wanted that repeater access and I never had it.)

Nand · Post by **Nand** » Sun Aug 24, 2003 4:51 am

wa2zdy wrote: As a younger ham, there was only one repeater in HT range - and it was as closed as the local bank on a Sunday. Thus even though I wanted to walk around town and be able to pull my trusty hand held out to chat, it wasn't fated to be. And it never did happen.

The above gives me an interesting idea for the amateur repeater councils to implement. Have some repeaters disabled on even days and others on odd days. This way we can have repeaters every 10 miles apart and not run out of frequencies. For future expansion, they can go to every three days etc. We could even have repeaters on the same frequency side by side. Every ham can now have its own repeater.

Nand.

wa2zdy · Post by **wa2zdy** » Sun Aug 24, 2003 8:36 am

Nand,

My experience described above was around 1977, 1978. The repeaters were just starting to sprout up everywhere as affordable synthesised rigs were still in the future. With the boom of the 80s and 90s, repeater coverage became nearly as universal as cell phone coverage is today.

Now that the repeater boom is in the past, there nearly are enough repeaters for every ham to have his own for private use. Here in central NJ, midway between Philadelphia and NYC, every pair on 2m has a repeater on it, yet there are few times one can actually find someone to talk with. It's a bizarre state of affairs to be sure.

That repeater from my youth has changed hands several times, but remains on the air from the same location. And it remains closed to "outside" users. Of course listening to it, there don't seem to be any "inside" users either!

KB9WZD · Post by **KB9WZD** » Sun Aug 24, 2003 4:43 pm

Is walking around with an HT up to your lips with a 49 inch antenna sticking up practical? Since you don't say, perhaps you're on UHF, and even that . . . it's funny looking at least.

It VHF and is about 49 inches its on a solid threaded connecter but its loading coil is flexible and that takes some of the strain but my icom ham hand helds BNC has not been so lucky it had a halfwave telescopic on it that didn't flex and ive had to replace it once and may have to again but is does give me vastly improved range

RapidCharger · Post by **RapidCharger** » Sun Aug 24, 2003 8:09 pm

Nand, where are you that you are "running out of frequencies"?

Would it be Southern CA, Chicagoland, or Florida?
Those are the only places I have ever heard of having really crowed ham bands.

There may be repeaters, but only a handful actually have users. There's a ton of 440 repeaters to use, but no one there to talk to!