Gain vs Vertical Beamwidth

[emtunderwood]

So, if a VHF P25 repeater is needing to talk a maxium of 20-25km away over terrain and the antenna is 218m above average terrain. Which one would potentially work better?

100w repeater 7/8 hardline mounted at 180ft on the tower.

6db omni (16deg vertical beamwidth)
9db offset (16deg vertical beamwidth)
8.5db offset (18deg vertical beamwidth)
6db offset (34deg vertical beamwidth)

 

[mmckenna]

The higher gain antenna is going to put more RF energy out towards the horizon. That'll improve transmit and receive out at the fringes.

In some cases, having that tighter beamwidth, can decrease performance closer in to the repeater site IF it is a very high level location (much greater than your 218 meters).

From what you are describing the higher gain antenna would be the preferred choice. But 25km range will depend on the surrounding terrain. Distance to horizon at 218m is up around 50km, but hills/mountains will block that.

 

[freddaniel]

If your antenna is at 180' on the tower and is at 218 meters or 715 feet above average terrain, then you are most likely on a 535' hill. Depending upon the need for communications at the base of the hill will dictate the angle of coverage. If you need coverage at the base of the hill, you can also include antenna down-tilt to fill-in that area. You should also consider the UHF path loss from the antenna at 1 mile is 90 dB, and 5 miles is 103 dB and 15 miles [aka 25km] is 113 dB. The main lobe of the antenna would be within 5 to 15 mile area, but the 1-5 mile area would have 10 dB less path loss to offset much of the null in the antenna pattern.
Any of the antennas would work OK, but the differences would be measurable, but most likely insignificant from the users perspective.

 

[kf8yk]

Are you sure your HAAT is 218 m (715 ft)? Under rule 90.205 you would need to reduce your ERP to 75 watts for a 24 km service area.

With 6-9 db antenna gains and 200 ft of 7/8" hardline your TX power will be limited to something like 10-20 watts depending on other system losses like a duplexer.

 

[prcguy]

If your antenna is at 180' on the tower and is at 218 meters or 715 feet above average terrain, then you are most likely on a 535' hill. Depending upon the need for communications at the base of the hill will dictate the angle of coverage. If you need coverage at the base of the hill, you can also include antenna down-tilt to fill-in that area. You should also consider the UHF path loss from the antenna at 1 mile is 90 dB, and 5 miles is 103 dB and 15 miles [aka 25km] is 113 dB. The main lobe of the antenna would be within 5 to 15 mile area, but the 1-5 mile area would have 10 dB less path loss to offset much of the null in the antenna pattern.
Any of the antennas would work OK, but the differences would be measurable, but most likely insignificant from the users perspective.

For a repeater on a 500ft hill a wide antenna beamwidth is really not needed for close in comms. Ive spent most of my life living at the base of an 1,100ft hill and have had many repeaters on top and at lower levels. At 1,000ft above average terrain it starts to be a concern with antenna mfrs recommending a little downtilt on a 10dBd UHF antenna and for roughly 6dBd gain and less no downtilt needed. I’ve run a DB420 9.8dBd antenna at about the 1,000ft level with no downtilt and had good handheld coverage around the base of the hill. I might add the shape of the hill or mountain will have more to do with close in coverage than height in most cases as parts of the hill or mountain can totally obstruct radio coverage. Not all hills are perfect “cones” with an antenna on top.

I‘ve also had commercial repeaters on various 5,000ft mountain tops in So Cal running 10dBd sticks with about 4 deg of downtilt and 6dBd omni with no downtilt and had no problems at the base of the mountains. Most if my customers ran handhelds but didn’t operate specifically up against the 5,000ft mountains. I did most of the close in testing myself driving to and from the sites.

I currently have a UHF amateur repeater at 5,000ft with a 9dBd master receive antenna and 6dBd transmit antenna, both with some factory recommended downtilt which I forget, and a 900 amateur repeater with 8.5dBd omni and about 4deg of downtilt and they are great at the base of the mountain on handhelds.

 

[buddrousa]

The OP asked about VHF not UHF

 

[emtunderwood]

Are you sure your HAAT is 218 m (715 ft)? Under rule 90.205 you would need to reduce your ERP to 75 watts for a 24 km service area.

With 6-9 db antenna gains and 200 ft of 7/8" hardline your TX power will be limited to something like 10-20 watts depending on other system losses like a duplexer.

When I became chief the FCC license was already in place. And just had another site coordinated and used the same 100w repeater even though we have power of 180 and erp of 180 and we are 315ft up a cell tower but only run 100w repeaters.

 

[prcguy]

The OP asked about VHF not UHF

The antenna gains and resulting beamwidth would be the same on VHF although you will only encounter gains like 9dBd with a 4-day dipole array set in line for 120 deg pattern. Its more likely to have less problems against the base of a hill with repeater on top due to lower gain antennas used on VHF.

 

[freddaniel]

If performance is what you seek, consider using either a second repeater receiver or a circular polarized antenna. See attached charts that show that any signal received with the signal polarization off by 45 degrees is down 3 dB and between 45 and 90 degrees it increases to at least 30 dB. Many studies have shown that only about 15% of the signals received are true vertical, and the vast majority vary between 0 and 90 degrees due to signal multi-path. In other words, only true visual line-of-sight is true vertical polarization, as everything else is multi-path, twisting and turning as you walk or drive, bouncing off buildings, signs, cars and everything out there.

Using a second diversity receiver is the secret of success for cellular systems. At first they used antennas spaced about 10 wavelengths apart, then later found cross-polarized antennas, like four stacked dipoles in an X pattern worked better. If your brand of equipment does not support satellite or second receivers, then a circular polarized antenna is your next best choice to consider.

Land mobile still operates in the proverbial stone ages, relying upon massive signal strength to overcome multi-path. Adding a satellite or second receiver will improve repeater input performance by 10 to 15 dB, mostly by eliminating almost all multi-path fading. This works for analog or digital systems. This is usually referred to as receiver voting. With analog, they use S/N or RSSI voting. With digital, the first receiver with a correct check-sum per packet is used.

This is way more important that trying to squeeze another 3 dB antenna gain out of your system.

 

[prcguy]

If performance is what you seek, consider using either a second repeater receiver or a circular polarized antenna. See attached charts that show that any signal received with the signal polarization off by 45 degrees is down 3 dB and between 45 and 90 degrees it increases to at least 30 dB. Many studies have shown that only about 15% of the signals received are true vertical, and the vast majority vary between 0 and 90 degrees due to signal multi-path. In other words, only true visual line-of-sight is true vertical polarization, as everything else is multi-path, twisting and turning as you walk or drive, bouncing off buildings, signs, cars and everything out there.

Using a second diversity receiver is the secret of success for cellular systems. At first they used antennas spaced about 10 wavelengths apart, then later found cross-polarized antennas, like four stacked dipoles in an X pattern worked better. If your brand of equipment does not support satellite or second receivers, then a circular polarized antenna is your next best choice to consider.

Land mobile still operates in the proverbial stone ages, relying upon massive signal strength to overcome multi-path. Adding a satellite or second receiver will improve repeater input performance by 10 to 15 dB, mostly by eliminating almost all multi-path fading. This works for analog or digital systems. This is usually referred to as receiver voting. With analog, they use S/N or RSSI voting. With digital, the first receiver with a correct check-sum per packet is used.

This is way more important that trying to squeeze another 3 dB antenna gain out of your system.

The problem is where do you find a good land mobile omni CP antenna? Something around 6 to 8DbIC on VHF or 8 to 10dBiC on UHF would be nice.

 

[freddaniel]

Well since Sinclair, RFS, ComProd and other mainline manufacturers have not taken the lead, it appears people must build their own.
The second or satellite receiver antenna can be any standard vertical antenna as it depends upon space diversity and not polarization diversity for operation. There are loads of CP antenna designs on the internet. The Cycloid Dipole is most common with FM broadcasters. The ham version is attached.

If you can use an offset dipole, making a vertical stacked set of four or eight "X", or crossed dipoles would be more practical for a shared site, due to the greater operating bandwidth. The antenna could have two feedlines and they could be combined, with one antenna having a extra quarter wave coax section the make it circular. See harness drawing attached.

 

[prcguy]

Well since Sinclair, RFS, ComProd and other mainline manufacturers have not taken the lead, it appears people must build their own.
The second or satellite receiver antenna can be any standard vertical antenna as it depends upon space diversity and not polarization diversity for operation. There are loads of CP antenna designs on the internet. The Cycloid Dipole is most common with FM broadcasters. The ham version is attached.

If you can use an offset dipole, making a vertical stacked set of four or eight "X", or crossed dipoles would be more practical for a shared site, due to the greater operating bandwidth. The antenna could have two feedlines and they could be combined, with one antenna having a extra quarter wave coax section the make it circular. See harness drawing attached.

I've made CP antennas in the past and I don't want to take on another big project. It would be nice to just buy an omni CP antenna with gain in either the commercial bands or amateur bands. And based on an antenna engineer I met who built a quadrifilar helix antenna for a repeater in Manhattan, RHCP seems to be preferred but I forget the reason why. The guy did some calculations to prove it and he said the repeater performed better than any other at ground level in the dense Manhattan concrete jungle.

I have another friend who had I believe Jampro make a custom single bay CP antenna for a 2m amateur repeater and worked wonders as the transmit antenna. He fed it 500w to make up for the lack of gain and loss going from CP to linear. A four bay of Jampro antennas for amateur or even commercial bands would be prohibitively expensive.

 

[freddaniel]

With the FCC limiting ERP below what is possible with a typical 100 watt transmitter, users should consider using CP as it will give them more "bang for the buck" in laying down signal without exceeding the ERP limit. The cost is usually 3 dB, but the transmitters are often operated at 50 watt or below anyway.

 

[emtunderwood]

CP antennas for the 150-160 range? Anything commercially made? We rent tower space and antennas have to approved by the owner.

 

[emtunderwood]

I really am starting to believe this topic as went in the wrong direction.. No mention of 220 or ham. It was a simple question of what would work better for longer ranges and cover the terrain with 4 antennas an their characteristics available.

 

[prcguy]

I really am starting to believe this topic as went in the wrong direction.. No mention of 220 or ham. It was a simple question of what would work better for longer ranges and cover the terrain with 4 antennas an their characteristics available.

You are correct. The bottom line in my opinion is at only 715ft above average terrain you should have no reason to go low gain for operation at the base of the hill and any of the antennas in post #1 should be ok with higher gains giving a little better coverage further out.