Make an antenna with RG-58 for a Repeater

[centuryvrproductions]

Nice setup... P25? I Just bought a Spectra Desktrac console and it came with a Spectra Digital 800 MHz Radio inside. I bought the Desktrac to use as a housing for my repeater (Maxtrac 50's) and I am going to use the Power supply inside to power them...

By the way, The desktrac came with a 15W Radio, Will the power supply be able to handle my 25W units? I'm not sure if its a 15W supply or if all Spectra Desktracs have the same, regardless of Radio Power...

 

[centuryvrproductions]

Antenna

Here is the antenna i an looking to get.

Tram UHF 406-512MHz Base Antenna 6.7db Gain 200 Watts | eBay

If you look at all of the losses I have calculated in the last posts, and how I am using 25W out at radio, with the 6.7db Gain, How much power should I expect to have at the antenna (ERP)

also how do i calculate that?

 

[zz0468]

...How much power should I expect to have at the antenna (ERP)

also how do i calculate that?

TX power - coax and duplexer loss + antenna gain = ERP.

Convert the TX power in watts to dbm (decibels referenced to 1 milliwatt), then you can just add and subtract the db values of everything else.

and for reference:
1/2 watt = 27 dbm
1 watt = 30 dbm
10 watts = 40 dbm.
100 watts = 50 dbm.

Your 25 watt transmitter = 44 dbm.

 

[centuryvrproductions]

TX power - coax and duplexer loss + antenna gain = ERP.

Convert the TX power in watts to dbm (decibels referenced to 1 milliwatt), then you can just add and subtract the db values of everything else.

and for reference:
1/2 watt = 27 dbm
1 watt = 30 dbm
10 watts = 40 dbm.
100 watts = 50 dbm.

Your 25 watt transmitter = 44 dbm.

What do I do with the 44dbm? say i have 44dbm - 0.83db loss - 1db loss + 5db Gain = ERP? (Example)

how do i add them? i can just add db + dbm?

 

[zz0468]

how do i add them? i can just add db + dbm?

Yep. DBM and db are essentially the same units. A db is a power ratio, a comparison between two power levels. 3 db, for example, means that one power level is half (or twice) that of another power level. Dbm is a ratio as well, but with a VERY SPECIFIC reference power, that being 1 milliwatt. Since the power difference between two 1 milliwatt signals is zero, 0 dbm is equal to 1 milliwatt. 44 dbm is 44 db higher than 0 dbm, 44 db higher than a milliwatt. Make sense?

So, just add them. That's the beauty of doing everything in DB. Loss is a negative number, so subtract that. When it's time to convert your ERP value in dbm to watts, a quick google search for a dbm to watt conversion chart or calculator will get you what you need. Or, better yet, search for the formula and calculate it yourself.

 

[centuryvrproductions]

Sweet, So with my current setup plan and equiptment, If I get an antenna with 6.7db Gain,

I will have 67.88 Watts ERP. From using a 25 watt transmitter

How is the recieve end going to work, will that be amped up along with the ERP?

 

[zz0468]

Sweet, So with my current setup plan and equiptment, If I get an antenna with 6.7db Gain,

I will have 67.88 Watts ERP. From using a 25 watt transmitter

Using your numbers, 44-0.8-1+6.7 = 48.87 dbm. That's an ERP of 77 watts.

How is the recieve end going to work, will that be amped up along with the ERP?

"Amped up"? :roll:

Antenna system gains and losses are reciprocal.

 

[prcguy]

Its probably a good time to throw out the antenna gain warnings before you calculate against meaningless dB figures.

For a reference, look at the major commercial antenna mfrs like Cellwave, DB Products, Sinclair, A/S, etc because they post the gain in dB over a dipole (dBD) and can be trusted.

Then consider some mfrs use dBi as a standard which is 2.14dB less than dBD. Then consider some mfrs make up numbers that are completely bogus in order to catch your attention and hopefully sell their sometimes inferior product.

dBi is a perfectly good gain reference to use as long as its disclosed in the advertising and you add or subtract the appropriate amount of gain when comparing to other known antennas. If there is no D or i after the dB gain figures you should be suspicious.

For example, the classic and trusted Cellwave Stationmaster is a 5dBD gain antenna and is about 7ft long at UHF. The absolute most gain you will find in a given size is an exposed dipole array and a 6dBD gain UHF version runs about 8 to 9ft long.

So, how does Tram get 6.7dB gain out of a 5ft 7in antenna? Its probably measured in dBi, or they invented the numbers which can skew your EIRP calculations. When shopping for antennas I like to compare the antenna type (collinear vs dipole array) to something similar from a known and trusted mfr to see if the numbers are believable.

There have been advances in antenna technology over the last 50yrs but not that much in the type you are looking at for your repeater. If the gain figures smell funny then they probably are.
prcguy

Here is the antenna i an looking to get.

Tram UHF 406-512MHz Base Antenna 6.7db Gain 200 Watts | eBay

If you look at all of the losses I have calculated in the last posts, and how I am using 25W out at radio, with the 6.7db Gain, How much power should I expect to have at the antenna (ERP)

also how do i calculate that?

 

[centuryvrproductions]

I Knew I would screw the math up, But its a better answer.

As for the antenna, what is a good one to use? I want the higest gain I can get. And it needs to be a dipole so I can fit it on my tower. Any suggestions?

Price Range up to around 80 Bucks..

 

[zz0468]

I Knew I would screw the math up, But its a better answer.

As for the antenna, what is a good one to use? I want the higest gain I can get. And it needs to be a dipole so I can fit it on my tower. Any suggestions?

Price Range up to around 80 Bucks..

Look... don't over do it, ok? You're going to be running an uncoordinated, probably poor running repeater, probably without an intermod panel, probably without narrow pass cavity filters, and who knows what the audio levels and transmitter deviation are going to look like - you've made no mention of any test equipment you have to set it up right.

Find a nice modest UHF ham antenna, forget getting the highest possible gain - you're not going to get that for 80 bucks anyway. You want to minimize the interference potential until you learn enough, and earn enough, to do it right.

 

[centuryvrproductions]

Look... don't over do it, ok? You're going to be running an uncoordinated, probably poor running repeater, probably without an intermod panel, probably without narrow pass cavity filters, and who knows what the audio levels and transmitter deviation are going to look like - you've made no mention of any test equipment you have to set it up right.

Find a nice modest UHF ham antenna, forget getting the highest possible gain - you're not going to get that for 80 bucks anyway. You want to minimize the interference potential until you learn enough, and earn enough, to do it right.

What I mean is I want to get the best antenna i can get at a price of 80 Bucks. I have no test equiptment but what I will do is turn the power down so i only have say 10 watts ERP. once I get coordinated, then i will be turning it up... But I wanted to get a good antenna with between 5-10db gain or so

 

[rescue161]

I recommend a DB 404 for a simple repeater. If you were local to me, I'd give you one as I have several. They would need to be modified as they are the "A" model, 406-420 MHz. I converted one and it works great on 462 (GMRS).

They just cost too much to ship.

You can make a Yagi for pretty cheap, but then you'll only get great coverage towards one area. When I changed the repeater frequency to GMRS, I changed to a Yagi antenna. I only wanted coverage towards a certain area. It worked great, but I lost a lot of coverage to the sides of the repeater.

 

[centuryvrproductions]

I recommend a DB 404 for a simple repeater. If you were local to me, I'd give you one as I have several. They would need to be modified as they are the "A" model, 406-420 MHz. I converted one and it works great on 462 (GMRS).

They just cost too much to ship.

You can make a Yagi for pretty cheap, but then you'll only get great coverage towards one area. When I changed the repeater frequency to GMRS, I changed to a Yagi antenna. I only wanted coverage towards a certain area. It worked great, but I lost a lot of coverage to the sides of the repeater.

I was looking to have coverage in all directions, Any suggestions to something like that? remember around $80.

I could make one for now but i need one with gain as im only running 25Watts + losses in lines

Also, would I need a preamp if i have 250W ERP, Does it recieve as well as it puts out at this power?

 

[rescue161]

250 Watts ERP is wishful thinking with only 25 Watts into a duplexer.

Even with 250 Watts, at 25 feet, you're still only get a few miles. More than likely, you're repeater will talk out farther than it will hear. You need to balance that factor.

Remember, height is your friend when it comes to UHF. My repeater was at 25 feet (~30-40 feet above average terrain) and I only got about 7 miles of hand-held coverage. Most of that was NOT inside buildings. Basically, if I walked into a building with any kind of rebar/concrete construction, I could not hit the repeater. Using mobile radios, I was able to hit the repeater for several miles out beyond the hand-held limits, usually to about 10-12 miles away, depending on the area. I wasn't concerned, as the area of operation of the repeater was only about an 8 mile area, so the compromise was ideal.

I almost forgot, all of the above coverage area limits were taken while using DES analog, which did decrease the range a bit.

 

[centuryvrproductions]

My antenna is going to be about 55ft above the ground. Not sure above sea level, but theres a fairly flat terrain around me... Using 25Watts and an antenna at 20ft got me about half mile around my house. (This was using 15ft of RG-58 coax and a homemade untuned coax dipole. And no duplexer of any sort) RX and TX

 

[rescue161]

The only reason I mentioned sea level, was because I was on the cosat. I was actually about 100 feet from the ocean. We were a lot higher than sea level, due to the terrain.

 

[centuryvrproductions]

If i get this antenna, will it work on 444.5 and 449.5MHz? It says 445-470MHz

Is it good?

UHF BASE STATION 5dB GAIN 445 - 470MHz ANTENNA | eBay

With this antenna, I should have an ERP of 45Watts. (From Calculations)

 

[LtDoc]

Will it work? Probably. The frequencies you quoted would be at one 'edge' of it's reported coverage range, so it may or may not be possible to tune the thing for them. I also have doubts about manufacturer's 'gain' claims when they don't tell you what that gain is in reference to (dBd - dBi).
- 'Doc

 

[centuryvrproductions]

Any recommendations to a high gain antenna for a price around $80 new or used? (not Directional)