Antenna for VHF Comms to HEMS helicopter

[Brentd2183]

KCH682 155.205 PL 67.0 is an existing site that was used for AMR ground communications that was then allowed to be our backup communications while flying. I've done propagations and I should have much better range than I am experiencing. My thoughts are the antenna is setup to look more down to the ground from the tower vs us up at 2000-4000 feet. I'm already asking around with our radio guys but wanted everyone elses thoughts. I'd figure whatever antenna are used by ATC would be where to start looking. I was halfway into the green on the east side of the station and still zero reception. It actually gets worse the higher you go, thinking
you are improving line of sight but I'm clearly climbing above the radiation pattern of the antenna.

you

 

[chief21]

KCH682 155.205 PL 67.0 is an existing site that was used for AMR ground communications that was then allowed to be our backup communications while flying. I've done propagations and I should have much better range than I am experiencing. My thoughts are the antenna is setup to look more down to the ground from the tower vs us up at 2000-4000 feet. I'm already asking around with our radio guys but wanted everyone elses thoughts. I'd figure whatever antenna are used by ATC would be where to start looking. I was halfway into the green on the east side of the station and still zero reception. It actually gets worse the higher you go, thinking
you are improving line of sight but I'm clearly climbing above the radiation pattern of the antenna.

youView attachment 147940

LMR (Land Mobile Radio) tend to use gain antennas to increase their coverage footprint. Gain antennas achieve additional range by compressing or focusing their signal more toward the horizon and/or a particular direction. If you could identify the LMR base antenna and the related aircraft antenna, you could check the product sheets to see if any gain is specified.

It would be logical that ATC antennas focus their signal upward and outward, rather than simply focusing toward the horizon. Be aware, however, that ATC antennas operate in a different frequency range than your commercial frequency, so using an ATC antenna would not work for you.

 

[jeepsandradios]

Most likely the antenna's have down tilt on them to help with subscribers on the ground. There are antenna's specifically used for A2G communications. A properly designed and implemented system is costly. Also using a ham radio propagation program may not accurately depict the system. Not sure what parameters you used for that study but accurate antenna specifications along with ERP and cable losses will change the coverage patterns greatly.

 

[Brentd2183]

Correct on the ATC description, I should have said the design style and propagation they use, not the frequency range. We do have proper radio techs looking into the issue and seeing what cost it would be to swap out the antenna at the site, but wanted to look here too. The site I used is just for general expectations and when dealing with other HEMS radio systems when I put in their proper specs it actually matches real world conditions pretty well, but of course our tech will use the real deal to give us a true expectation. It's an MTR2000 110 watt that's feeding the antenna.

 

[kayn1n32008]

You want an antenna with low gain, like a single folded dipole.

 

[mmckenna]

We have a site at work that has a VHF air band transceiver used to talk to one of the local air ambulances. It's on a mountain at about 4,000 feet. Not sure what their coverage is, but It's probably pretty good. It's not mine, so I don't know exactly what they are running.

As for the antenna, I'm not sure of the exact brand/model, but it looked a lot like a 1/4 wave coaxial:

Kreco Antennas - High Band Co-Axial Antennas

As kayn1n said, low gain is what you want. Usually antennas designed for LMR (non-aircraft coverage) is higher gain/narrow beamwidth with maybe a bit of downtilt.

I think your propagation software is probably being overly generous, also.

 

[littona]

When's the last time the base station radio was PMI'd? Has the aircraft radio been checked out? As a radio tech with another HEMS company, I find the aircraft radios never get an alignment after they leave the factory. The AMT's don't have the equipment/training to do it. Focus on the easy stuff before replacing the antenna.

 

[Brentd2183]

Radios in aircraft are brand new with new antennas and work well with all other systems we talk on including other VHF HEMS ground communications. Radio shop will be heading out to do a full check over all the equipment to make sure everything is operating the way it should be before replacing the antenna. But we do notice it works far better when we are tree top level climbing out vs having a solid amount of altitude so we really think its high gain and looking down or close to the horizon vs pushing out

 

[kayn1n32008]

we do notice it works far better when we are tree top level climbing out vs having a solid amount of altitude

If that repeater anyenn is any more than 3dBd gain, it's vertical beamwidth is going to be quite narrow.

Try and find out the make/model of the repeaters antenna.