Transmitter Isocoupler to Co-Locate 3kW ERP FM on a 4kW Hot AM Tower

I'm Paul Hollings and one of my jobs at Aareff Systems is to come up with cost effective solutions to customer requests. I was asked by a client based in the islands of Micronesia if it was possible to put and operate FM antennas on an existing LIVE or HOT base insulated 4kW AM tower. I said I believe it is, but I've never done this before, so give me a week and I'll let you know. I started brainstorming.

Isocoupler Design

The first thing that occurred to me was capacitively coupling the FM signal to the AM tower using a high pass L network. I was in no doubt this would work on the hot center connection of the FM coaxial feedline, but how could the grounded outer connection of the FM coaxial line be connected to the HOT AM tower? that was the tricky bit. I mulled it over in my mind over lunch and figured I could eliminate the FM coaxial ground using an 800 watt Balun to form a (+) Hot FM signal and a second 800 watt Balun to form a(-) Hot FM signal which could be both passed through separate capacitively coupled high pass L networks.

If I made two of these 800 watt Baluns and put them back to back I could do the following:

Coaxial Unbalanced (Transmitter Side) --> Capacitor Coupled Balanced --> Coaxial Unbalanced (Tower Side)

I came up with the following idea. Now I needed to calculate the values.

Schematic diagram showing an AM and FM Isocoupler design using back to back high pass RF L networks with Wilkinson splitters / combiners in a balanced arrangement

C1 and C2

I thought it would be prudent to start with the main capacitors and then work everything around them. In the workshop I had some aluminum plates cut for use as antenna brackets that were 100 mm square and 3.2 mm thick. If I used two of these as capacitor plates I calculated I could get 32pF, this is in the right ballpark for a VHF high pass L network.

L1, L2, L3 and L4

I calculated a value of 80nH for each side of the 32pF capacitor to form a π high pass network or two high pass L networks back-to-back that would resonate and pass 88.3 MHz. I simulated all this in MC12 with a 50 ohm source and load and it worked perfect with a moderate Q factor. If the Q factor was too high, then the bandwidth would be too tight and narrow. Given that this was going to be mounted outside on a tower, a narrow bandwidth would be detuned easily with ambient temperature variations. It was vital the Q factor was low enough to prevent this being a problem.

T1, T2, T3 and T4

Now I had to split the coaxial line in to two lines to feed the two π high pass networks. A Wilkinson splitter/combiner (without the balancing load) appeared to be the solution. Two RG11 1/4 wave lines of 75 ohms were used to produce two equal phase and amplitude outputs of just over 50 ohms. I needed balanced phases, basically 180 degrees between them, not the same phase. I simply added to one of the RG11 1/4 wave lines and additional 1/2 wave line. This resulted in two Wilkinson splitters balanced in phase that could be connected directly to each side of the two π high pass networks. This arrangement was simulated in MC12 with excellent results, good bandwidth and close to 0dB passing through at 88.3 MHz using a source and load of 50 ohms. The simulation showed -69dB through rejection at 1449 KHz the clients AM frequency.

Isocoupler Construction

The drilling, sawing, cutting etc. started and to cut a long story short I came up with the following.

Physical design and construction of an AM and FM Isocoupler design using back to back high pass RF L networks with Wilkinson splitters / combiners in a balanced arrangement

Isocoupler Testing

I connected the Isocoupler up to the antenna analyzer on one end and a precision dummy load on the other end. The copper coils could be stretched or compressed in length by moving them along the stainless steel threaded bar. This is the fine tuning shown in the picture. With this adjustment I was able to get a VSWR of less than 1.1, just about perfect.

Whilst making these tests I put the lid of the enclosure on, no effect on the SWR. I also touched the copper coils and capacitor plates, this had marginal effect on the VSWR raising it to 1.2.

Next it was time for the power test. I connected the output of a 1kW amplifier to one end and the other end to a high powered dummy load and fired it up. Again, it worked perfectly, as far as the 1kW amplifier was concerned, the Isocoupler was not even there, it was transparent to the 1kW.

I left it powered up with 1kW going through it for 30 mins. Again, I put the lid on to see if it would have any effect on the 1kW or reflected power, nothing it remained perfectly stable. After 30 mins I turned off the 1kW and quickly touched the coils and capacitor plates to see if I could feel any heat, nothing it was stone cold. The black resin fills, RG11 and connectors were also cold unaffected by the RF.

Despite the enclosure being IP67 standard, just be on the safe side to prevent future galvanic corrosion the copper coils and soldered joints were painted with the electronic grade resin and then the unit was retested at 1kW.

Isocoupler Materials

■ ABS UV Resistant IP67 Electrical Enclosure
■ Stainless Steel Screws, Washers and Nuts
■ Aluminum Channel
■ Copper Tubing
■ Teflon
■ Nickle Plated Connectors
■ Solder
■ RG11 Coaxial Cable
■ Electronic Grade Epoxy Resin

AM Ground System Coupling

The AM signal suppression of 69dB to from the Isocoupler mounted at the bottom of the AM tower is wonderful, but it occurred to me that if the FM coaxial cable was laid on the ground between the AM tower and the transmitter building, the AM signal from the AM ground system could also be easily coupled into the outside of the FM coaxial cable. This would result in a potentially strong AM signal getting to the chassis of the FM transmitter causing multiple problems. To eliminate this possibility, I decided on a second identical Isocoupler to give an additional 69dB of suppression to the AM signal. This second Isocoupler would be installed where the FM coaxial cable enters the building close to the connection of the FM transmitter.

The complete arrangement with the two Isocouplers is shown below

Full set up of a 3.2kW ERP FM transmitter and antenna system installed on a HOT AM 4kW tower using a balanced Isocoupler

Full 3.2kW FM System Test

It was impossible for me to power test the 4kW AM part of the system at the workshops, but I was able to do a full system power test of the FM side of the system. The four way dipole stack was installed on our test mast and the first 50 ohm coaxial cable was connected between the antenna and the first Isocoupler. Then the second 50m coaxial was connected between the first and second Isocoupler. Finally, the connection from the second Isocoupler to the clients 800 watt transmitter.

I powered it all up and Murphy's law did not punish me, everything worked great as expected. In some attempt to test the AM side I shut down and disconnected the FM transmitter. Then I went to the FM antennas on the test mast, disconnected them at the splitter and connected a 1 watt signal generator of 1449 KHz to the FM coaxial cable going back down. I returned back down to the FM transmitter to the other end of the coaxial cable, plugged it into the analyzer and looked for the 1449 KHz signal. There was no trace of it, nada. It appeared that the suppression of 69dB +69dB (total of 138dB) was working.


I felt confident at this point that it would work with the clients 4kW AM transmitter and tower and I could disassemble the whole system set up, clean it up and pack it for the client, it was a wonderful moment. The guys at Aareff took care of this and shipped it.

A couple of months passed and then finally the client made the install with local technicians. We got the good news that it was all up and running on AM and FM at full 100% power and the FM audio quality was amazing. It's been this way now for six months and everything is still working great.

I'd like to thank all the guys at Aareff for the great work they did in building the Isocouplers with me and a special thanks to Alex and his team in Micronesia for a first class installation. Maybe you have an AM station or you know somebody that could benefit from these Isocouplers, if so, tell me what you need, talk to me!