As I live in apartment building I have apparent problem with setting up ham radio antenna. The obvious place to set up antenna was the roof as it is easily accessible. Due to small space and me not wanting to make mess of a wires, I decided to use compromise vertical multiband. A friend had spare DunaX-GPN4B (40, 20, 15 and 10m) so it was obvious choice. I had to put down about 30m of coax cable to my apartment, and soon I was in the air.
Not for long I found out that I do miss 80 meter band a lot. Also I noticed that WARC bands are also very interesting. Tried to overcome that by using antenna tuner with so-so results.
It was clear I had to put up some more antennas. I am considering some kind of dipole or end fed wire antenna for 80 m and probably another multiband vertical for WARC bands.
Two more antennas – two more 30 m coaxial cables. No way! There are two good reasons: one, for that length cable has to be of high quality, that means high cost, and for two, as I do have quite fine and tolerable neighbors, I do not want to test their patience having that many cables hanging from the roof.
The only viable course of action is using antenna switch on the roof. Again, as commercial ones are far too expensive, it had to be built. But, hams are born to build things, right?
Ideas come from necessities, and it was not hard to figure out what has to be done. I’ve set limits to have just two antenna cables already going to the roof – one for HF and one for VHF/UHF. Switch should have two functions, to allow me select one of three HF antennas.
Switch has to go on the roof and to be controlled remotely using third, thin signal cable. Remote control unit should sit on top of my Ft-857D.
I also had an idea who will do the hard work for me: YU1ZZ (Milan) already planned to do similar build for himself so I knew he would be interested to get involved, and YU3ACO (Aco) is very skilled, has workshop, tools and most importantly, does not know to say no when asked :). Mine job was to give initiative, think up details and obtain necessary parts.
1. YU1ZZ suggested that we should use relay OMRON G2R-1, but I decided to try cheap ready made board with four relays driven by transistors. It was meant to be used for various micro controllers, but it seemed usable for antenna switch too. I liked idea of driving relays with transistors as that meant less current needed to go through cable.
Original idea was to use switch for three HF antennas only, but board I got had four relays, so we decided to use fourth one as UHF/VHF switch. Relay switching contacts were declared for 10 Amps so we expected the to handle even much more than 100 W which my FT-857D can produce.
2. HF antennas should be earthed when not in use to prevent static electricity to build up.
3. Power supply should provide 12V to 13V, and actually use the same power source as radio station. FT-857D already has connector with power output for external devices, so switch could even power up together with radio.
4. We would use shielded UTP cable for signaling. It has 8 wires. Four are needed to command relays, and other four are used for powering up switch (two wires in parallel to lower resistance). Total cable length needed is 35 meters.
5. It was planned to use LM7805 voltage regulator as planned relay board was designed for 5V powering. This also deals with possible voltage drop through lengthy cable.
As we were not sure if antenna switch would be usable at the end, we decided to look at project as on prototype. We did not pay much attention to looks, but to try to get needed functionality. Once we find out if it can be used, it is not problem to make it all over again, optimized in functionality and better looking.
The first issue was housing for the switch. YU3ACO found out top cover of some long time dead CD player, and with scissors and some drilling made something that looked like a box. Looks was not an issue, more important was that it was metal and shielded. It is not weather proof but it does not have to be. It will be mounted in bigger weather proofed plastic case anyways.
Remote Control unit was made of housing for dead LED switching power supply. It actually matched dimensions of Ft-857D pretty well. Anyways, it was good enough to fix two rotary switches that are controlling relays power connector and LED to display active antennas.
We used CAT6a shielded UTP cable and two sets of 8 pin connectors salvaged from some old Russian radio equipment (which seemed much tougher that RJ45 we initially thought to use) for connection between Remote Control unit and Antenna Switch.
After some hours of work it was ready made and could go to testing department. That means I had to bring it to YU1ZZ to test it with MFJ antenna analyzer and if that goes well, to attach it to his rig and antennas.
The worst problem we could meet was if we get high SWR in antenna switch. Relays we planned to use were not coaxial antenna relays, and contacts in them were not supposed to be used for high voltage and high frequency so we did expect there might be problems.
This showed as not so much of an issue. Actually, for HF, relays worked very well. We tested it only with 100 W output power but it should work well with much more RF power. On VHF frequencies it was not so good. SWR showed up (but that is not necessarily because of relays).
We installed switch to YU1ZZ equipment and the first results were very nice. Switch worked fine, and we were happy.
But then Milan realized he was testing with low power. As soon as he powered up rig to 100 Watts, we had serious problem: relays were acting uncontrollably on higher HF bands. RF induction inside switch was high enough to trigger transistors and relays. Sad thing was that we could not avoid it. Shortening wires and even using ferrite chokes did not help.
That meant that my idea of using ready made relay board had to be abandoned. We had to go with using plain relays as YU1ZZ initially suggested. So, relay board was taken out and replaced with OMRON G2R-1.
Relay for VHF/UHF was installed first. It was linked to antenna connectors as tight as possible. SWR was lower but still not good enough. Shielding relay with thin Aluminum helped a lot but SWR was still there. It was not unusable but nevertheless to high for our taste.
Three relays for HF switching were to big to be fixed near antenna connectors so they were connected using short pieces of RG58 cable. They were also placed in shielding Aluminum box just in case although that was not necessary.
New RF test showed no problems. Antenna switch acted very good in all HF bands even with full 100 Watts of output power passing through. On VHF, SWR introduced by antenna switch was much lower, but still too high to our liking. That fourth relay will be replaced with proper coaxial switch.
Now, when all is done, we could provide schematics for others to use.
All relays are triggered by GND. That allows power voltage for whole unit to be regulated with single voltage regulator. Schematic shows LM7810 as we tested relays to work reliable on 10V. However, relays do not require large current so voltage drop even on such long cable is not big. We actually used LM7812.
Relays have one normally open and one normally closed contact. Antenna goes to common contact, and when relay is inactive it is earthed through normally closed contact to discharge eventual static electricity. When relay it active, antenna is connected to coaxial cable that goes to radio station.
Remote control unit is very plain. It powers up from external power source of 12V to 13.8V. Has two mechanical rotary switches that control two group of antenna inputs (A for HF and B for VHF) by sending GND to activate relays. LEDs are used to display switch states.