How much gain do I want?

[radioman2001]

Sorry for the partial post above, something went wrong while posting.

Antenna design is a combination of theory and science and witchcraft. I have had some really weird things happen with hi gain antennas, like being right underneath a 10db UHF station master on a 70 story building and not being able to talk in.

I did some calculations with the following 50 watts out of transmitter, and 50 ft LMR400 1.35 db loss, 3db for connectors, 2db for duplexer, add another 1.5 db for misc losses. With a real 5db antenna at 50 ft elevation I ran a plot and discounting obstructions (flat terrain) you will get appx 7-10mi. With the 5db antenna you basically negate all your losses up to the antenna, and with a 10db antenna you will only gain about 2-3 miles. If you provide Lat Long I can produce some real predictions from your location.

Doing some more calculations, 50 watts to the antenna gains you about 2-3 miles, just like the gain from the 10db antenna. Are your near an airport? and is that why you can't go above 50ft. Maybe consider going in a tall tree, as being below the trees are going to attenuate your signal a lot. Also stay away from that 300ohm ladder antenna, for $44.00 you don't even get the pipe. You could build one yourself for about $10.00.

At 65 ft you get another mile,at 100ft you get another 3-5 miles. Height is your friend, so you might consider placing your repeater on a building in the area if it's higher than your home. With going on a building you also gain some of the losses from the coax run.

 

[Ubbe]

Antenna design is a combination of theory and science and witchcraft. I have had some really weird things happen with hi gain antennas, like being right underneath a 10db UHF station master on a 70 story building and not being able to talk in.

That is wierd. Antennas usually have some stray directional loobs going in a downward angle that should take care of direct nearfield coverage, but most coverage at street level are problaby from reflections off other buildings. I only have experiance from 30 story buildings and 10dB antennas and that even worked satisfactory inside the building at ground floor. Of course all the other transmitters on the roof will totally block any portable receiver that aren't of the highest quality.

Telewave 6dBd vertical:

4-stacked 8dBd dipoles:

/Ubbe

 

[DylanMadigan]

A little more information may be helpful in evaluating your options.

What is the length of LMR-400 cable between the repeater and the antenna?
What is the make/model of your repeater?
What is the make/model of the combiner you are using?

75ft of LMR-400
Vertex VXR-7000
If a combiner is a duplexor, idk some $50 from ebay.

I do wish to upgrade all of these in time. My idea would be replace this stuff with better equipment as I get money and/or find people selling used stuff cheap, then I can use my current equipment to build a decent mobile repeater that wouldn't need much coverage (compared to our main repeater).

Are your near an airport?

No the town says they regulate anything 50ft and higher (so I'm really 49), or I'd need to pay for permits and annual inspections and what not.

Also being the trees are like 40-45ft on average, I'd imagine at the base of the tower the signal would reflect off of the tree tops so I don't shadow the site. I mean don't get me wrong I don't really know what I'm talking about lol just doing my best educated guess. I try to think of radio waves as a different form of light (because I guess it really is).

 

[prcguy]

Being at the base of a 70 story building with a 10dBd ant on top is unfortunately a real life situation and you will have lots of dead spots. Antenna mfrs usually recommend some downtilt for a 10dBd gain antenna when its on a 1,000ft or taller hill but in that case the base of the hill is some good distance away from the antenna and dead spots are minimal. A 70 story building would need something to supplement inside building communications.

In that case here in Los Angeles where buildings are much shorter I might mount a 10dB Yagi high on the roof and point it straight down or wire the building with leaky line radiating coax, either one fed from a splitter or coupler off the main antenna. Worst case would be a trunk line running the length of the building with couplers every floor or few floors feeding separate antennas for each floor or area. That gets expensive real quick. If you don't want to loose power to the main omni on the roof you might tap a little power then run a separate amplifier to feed the inside building antenna.

I see you listed 3dB for connector loss. How many connectors are there in the system? I would expect about .1dB loss per N connector max at UHF so you might have added more loss than needed.

Sorry for the partial post above, something went wrong while posting.

Antenna design is a combination of theory and science and witchcraft. I have had some really weird things happen with hi gain antennas, like being right underneath a 10db UHF station master on a 70 story building and not being able to talk in.

I did some calculations with the following 50 watts out of transmitter, and 50 ft LMR400 1.35 db loss, 3db for connectors, 2db for duplexer, add another 1.5 db for misc losses. With a real 5db antenna at 50 ft elevation I ran a plot and discounting obstructions (flat terrain) you will get appx 7-10mi. With the 5db antenna you basically negate all your losses up to the antenna, and with a 10db antenna you will only gain about 2-3 miles. If you provide Lat Long I can produce some real predictions from your location.

Doing some more calculations, 50 watts to the antenna gains you about 2-3 miles, just like the gain from the 10db antenna. Are your near an airport? and is that why you can't go above 50ft. Maybe consider going in a tall tree, as being below the trees are going to attenuate your signal a lot. Also stay away from that 300ohm ladder antenna, for $44.00 you don't even get the pipe. You could build one yourself for about $10.00.

At 65 ft you get another mile,at 100ft you get another 3-5 miles. Height is your friend, so you might consider placing your repeater on a building in the area if it's higher than your home. With going on a building you also gain some of the losses from the coax run.

 

[radioman2001]

The connector loss was calculated from real world testing using an Anritsu. I calibrate and loss factor all my cables for tuning duplexers etc, and this is what I see with the PL-259. The most common connector out there, and the "N" will be a bit better since it is a real 50ohm connector unlike the PL which is 70ohm.
Lets start with the jumper from the hard line to the antenna, at the other end you will have a jumper from the hard line to the lightning arrestor, then from the arrestor to the duplexer, and finally to the TX or RX. You are also not going to use LDF4 cable for jumpers so there is all this loss.
As far as the 10db stick on a tall building, yea we use down tilt if there are no other buildings reflecting the signals. When on a tall tower with no surrounding sources of reflection you can run into this issue.

To DylanMadigan, stay away from LMR400 (I only included it since that was being used) since it will cause issues with duplex operation after a few years from corrosion and stay way from CCR duplexers. Instead buy a good used Bird, PD, Motorola or other.

 

[prcguy]

So the 3dB connector loss is actually jumper losses?

The connector loss was calculated from real world testing using an Anritsu. I calibrate and loss factor all my cables for tuning duplexers etc, and this is what I see with the PL-259. The most common connector out there, and the "N" will be a bit better since it is a real 50ohm connector unlike the PL which is 70ohm.
Lets start with the jumper from the hard line to the antenna, at the other end you will have a jumper from the hard line to the lightning arrestor, then from the arrestor to the duplexer, and finally to the TX or RX. You are also not going to use LDF4 cable for jumpers so there is all this loss.
As far as the 10db stick on a tall building, yea we use down tilt if there are no other buildings reflecting the signals. When on a tall tower with no surrounding sources of reflection you can run into this issue.

To DylanMadigan, stay away from LMR400 (I only included it since that was being used) since it will cause issues with duplex operation after a few years from corrosion and stay way from CCR duplexers. Instead buy a good used Bird, PD, Motorola or other.

 

[bchappuie]

All the questions about what gain antenna to buy, there are license limts on ERP, so based on output of radio, you need to figure out what gain antenna acheives the licensed ERP. Takes some math work to include line losses with antenna gain to determine the radio output limits.

 

[radioman2001]

Connector losses can be over 1.5 db depending on brand of connector, for jumpers it will be what coax you use and how good you are at making the connections. So one jumper made with RG8 (Brand effects loss so again too many variables) with 2 ea PL-259 (Phenolic) can give you 3db losses. Using the same connectors with RG58 you are going get even more loss, because of the RG174 adapter. Use LDF4 connectors and you are more likely to see .25db loss per.
If you use a good silver and Teflon connector with RG-216 coax you can get down to around 1.25 to 2db. Again there is too many variables, that's why I test each cable with the Anritsu. When I make cables for duplexers I try for no more than .9 db (at 160mhz) with either RG400 or RG216.
All my test cables are marked with their loss factors on a tag so I can acurately know how much duplexer loss I have vs the total seen with the test cables in the line. The Mot T1500 series duplexer with PL's has been mentioned, but have you ever seen how well they are made? Silver Teflon PL-259 using RG400 silver solid center and silver shield coax. Plus they are only a few inches long.

As far as gain you can go with more gain than you need and turn the power down. You will gain more RX sensitivity with a higher gain antenna. My preferences for antenna's are based on how high the antenna is going to be mounted, how far and where I want to cover as there is no need to listen to users outside my area. Then based on the gain figures of the antenna, then deduct coax and connector losses (confirmed by Anritsu) I adjust my power up or down to meet FCC ERP limits. Most repeaters are not going to exceed power output which is usually a max of 110 watts at the antenna port, but GMRS is lower.

I found this chart, should be self explanatory on connectors.
'UHF' Connector Test Results

After going through these charts, I might consider changing the connectors on some my duplexers with the PL-259's with N which seems the way to go.

 

[Ubbe]

You can use any connector type except PL259 above 50MHz. All the charts show zero loss with sma-bnc-N-tnc. Probably F connectors are equally good as those are used at GHz frequencies. Only PL/SO types have measurable losses and should only be used at HF frequencies.

/Ubbe

 

[radioman2001]

Quote"
You can use any connector type except PL259 above 50MHz.

Totally what the test results say. For some radios, duplexers, and antenna's it's not that simple to change. At the very least buy good Amphenol silver teflon PL-259 connectors.

 

[prcguy]

I've measured a lot of connector loss in my time and have never seen over about .1dB loss on an N connector at 500Mhz and not much more at 1GHz. Silver Teflon PL-259/SO-239s are just fine at 500MHz. Take a look at Motorola manufactured UHF duplexers, they have four cavities with two connectors each for a total of four PL-259s plus a T adapter at 500MHz in both the transmit and receive lines and the insertion loss spec is no worse than any other similar duplexer.

Quote"
You can use any connector type except PL259 above 50MHz.

Totally what the test results say. For some radios, duplexers, and antenna's it's not that simple to change. At the very least buy good Amphenol silver teflon PL-259 connectors.

 

[radioman2001]

Then you are fortunate for the low losses.