2m antenna on 1300Hz at 1000w am station tower with 104.9 MHz FM

[FKimble]

Our radio club has a possible chance to install a 2m repeater on a local church tower, but I am told it has an AM station at 1300Hz and an FM simulcast at 104.9MHz. Does this present any different challenges than a regular tower? Have not seen the site in years but at one time it was local city PD tower and we had a 2m repeater on it. Any info greatly appreciated.

Frank

 

[WB5UOM]

It is doable, but has to be done in a specific manner.
The antenna heliax has to be grounded to the tower at certain spacing, you need a rf isolator mounted at ground level and the heliax off the tower connects to its top side (it has to be bonded to tower too), then the heliax to your repeater connects to bottom side of isolator.
Will require radio station engineer to verify that the installation does not change the tuning of the AM antenna which IS the tower itself.
it is NOT a weekend ham radio group install.

 

[G7RUX]

the medium wave station is the one that will cause the most awkward issues since MF will get in to everything everywhere if you aren't careful. The FM station shouldn't be too bad to avoid issues with and a modest filter should avoid most of the issues for that one.

As already mentioned, grounding and bonding is absolutely key to avoiding issues with the MF Tx getting in, and don't forget power leads and feeders which make excellent antennas at MF and are often overlooked.

 

[FKimble]

A little more info: when we had the 2m there approx 5 years ago it was a "radio" station, not sure of band or frequency. There were 3 antennas near the top, we were told they were available to us but they were going to use one soon. I think for one of their departments. We checked them and used closest one to our freq, worked fine. I am assuming and hoping the antennas are still there complete with hardline and required grounding. Otherwise it will be totally out of our price range. Did I mention we are are pooooor ham club!

Frank

 

[JustinWHT]

I've installed over a dozen WISP (Wireless ISP using license free WiFi UNII channels) access points on AM towers.

There are two types of AM towers - older towers were isolated from the ground by a ceramic insulator or grounded and used a RF isolating shunt feed. From your description this was a typical grounded tower that was repurposed as a shunt-fed AM site.

As the site already had radio repeaters, the AM broadcast engineering study would have included that information necessary for the FCC license, so no need (other that letting their broadcast engineer of record - this would be included in any FCC correspondence - knows your plans) to recertify anything.

Now moving on to best practices. AM stations using amplitude modulation can creep in to everything - any commercial grade repeater repurposed for 2M FM would be clean enough, but for good measure add additional grounds to your coax line at the bottom (its already bonded to the tower at the top via the antenna mounting hardware and no need for interstitial bonding) and radio cabinet and bond them to the tower.

As for repurposing any existing VHF antenna - those antennas are usually a Decibel Products DB224 tuned for entire 152-158 MHz band. When fed by 144-148 MHz transmitter there would be a slight SWR increase but acceptable.

if this is a galvanized metal tower such as the Rohn 25, 45, or 55 you should never directly bond any copper (including copper shealth coax) to the galvanized metal. Use stainless steel bolts, washers, and nuts to galvanicly isolate.