ERP Calculation help

911-EMT · Post by **911-EMT** » Thu Mar 23, 2006 9:34 am

We have FCC license on VHF for 110 watt on the power output, and 225 watt on the ERP, here is our site setup, we use a 110 watt radio, a 3db gain omnidirectional fiberglass antenna, 150’ ft. of lmr600, the building the antenna is mounted on is 51.0 meters high, and 8.2 meters amsl. Now I need help calculating the ERP if we are at 225 watt with the above setup? If not how do we get the ERP to 225 watt.

k2hz · Post by **k2hz** » Thu Mar 23, 2006 10:21 am

Someone did a sloppy job of calculating your ERP. If the TX output is 110 and the antenna gain is 3db the maximum possible ERP would be 220W if there was no feedline loss. Your loss in 150' of LMR 600 would be approximately 1.5 plus allow another 0.5 db for connectors and jumper losses.

I would say your net gain is 1 db (3 antenna - 2 feedline and connectors)so actual ERP is about 150W.

The only solution would be to get a higher gain antenna but I would say it is not worth changing anything. You will never notice the 2 db difference.

JAYMZ · Post by **JAYMZ** » Thu Mar 23, 2006 12:53 pm

If you have all the info on your gains and losses I made a quick and dirty ERP calculation worksheet where it does the math for you. Just plug in the info from 2 charts and you are good to go.

http://www.k2jsv.com/jaymz/erp.xls

Using that, it would appear that your ERP is about 150 watts. Like k2hz said, your maximum is below what you are licensed for. Maybe they "over-estimated" your ERP for your license.

911-EMT · Post by **911-EMT** » Thu Mar 23, 2006 1:51 pm

Thank you all for the info, so now is my question what would be the simples way to get the outer 75 watt?

JAYMZ · Post by **JAYMZ** » Thu Mar 23, 2006 2:19 pm

More powerful transmitter, lower loss cables, higher gain antenna like a DB224 which has 6dB gain. It also depends on your topography as well. You may not need to have a higher ERP.

k2hz · Post by **k2hz** » Thu Mar 23, 2006 3:49 pm

As I mentioned in my prior reply, an additional 75 Watts ERP will not make any noticeable difference. Why is this a concern????

A 2db improvement on a noisy signal would be discernible only on test equipment. You would need about a 6 to 10 db improvement to really notice a change.

What is the problem with your system that you want to correct??

911-EMT · Post by **911-EMT** » Thu Mar 23, 2006 5:41 pm

We are operating on a vhf simplex frequency in the city, with all the tall buildings giving us a hard time on the vhf band, we plan moving to uhf band but in the mean time we would like to get the max out of the licenses, we do have a problem in the RX more then TX, like receiving a 5 watt portable radio in the 5 mile range of our FB station.

k2hz · Post by **k2hz** » Thu Mar 23, 2006 6:20 pm

911-EMT wrote:we do have a problem in the RX more then TX, like receiving a 5 watt portable radio in the 5 mile range of our FB station.

That is what I suspected and why increasing base TX ERP is way down on the list of things to do for most two-way system coverage problems.

Most systems are overdesigned on base station ERP to begin with. The base to mobile range far exceeds mobile talk back.

An improved antenna does help both TX and RX coverage so it is always something to consider but there are trade-offs in this soultion. Antenna gain is acheived either by a directional antenna pattern in which case you loose in one direction what you gain in another so it helps only if the base is not in the center of your coverage area. The other soultion is an antenna that produces omnidirectional gain by concentrating signal toward the horizon. This can cause the signal to overshoot areas near the base station resulting in dead spots or if "downtilt" is used to compensate for this then distant coverage suffers.

My main point is that a few db of improvement is meaningless and will not be noticed on TX or RX range. If you spend a lot of money on a better feedline, you could only see an improvement of about 0.5 db which is barely measurable.

So, the best option, which is a 6db gain antenna, would only give a very slight improvement and could make things worse. In an urban environment, UHF should help. If you are going to spend the money for a new system, get a vendor that will do some good engineering of an appropriate antenna system and look at your proposed antenna site to see if it satisfactory for the coverage you want. Like real estate, it can be all about "location, location, location" for your base antenna vs the desired coverage area rather than try to make up for a bad site with high power.

wa2zdy · Post by **wa2zdy** » Thu Mar 23, 2006 6:37 pm

JAYMZ wrote:More powerful transmitter, lower loss cables, higher gain antenna like a DB224 which has 6dB gain. It also depends on your topography as well. You may not need to have a higher ERP.

They're already at 110w so more power is out of the question. And 2dB is not worth the trouble. And if they're moving to UHF soon, how much more infrastructure investment on high band can be justified?

JAYMZ · Post by **JAYMZ** » Thu Mar 23, 2006 8:09 pm

wa2zdy wrote:
JAYMZ wrote:More powerful transmitter, lower loss cables, higher gain antenna like a DB224 which has 6dB gain. It also depends on your topography as well. You may not need to have a higher ERP.
They're already at 110w so more power is out of the question. And 2dB is not worth the trouble. And if they're moving to UHF soon, how much more infrastructure investment on high band can be justified?

You are right. Just answering the question as it was posted. It had already been stated that the difference is inconsequential. But if he REALLY wanted to go down the road, those are the points that would need to be changed out for a miniscule difference.

For me personally, my repeater is tuned back so the coverage of the repeater does not out distance what a mobile unit could talk back into. 120 watt ERP works just fine for me. Higher power is not always the answer.