Antenna Switch
I have the need for a simple inexpensive two position remote antenna switch. This switch will be responsible for switching between a dual band 2m/70cm vertical and a pair of 2m and 70cm beams on my latest tower, also known as Tower #2 on my website. After looking at some commercially available ones I realized I wasn't finding exactly what I wanted so I set out to build my own.
I had some requirements in mind.
1. It must be a two position switch
2. It should work well at VHF/UHF frequencies
3. It should be made of fairly common components
4. It should be fairly inexpensive
Relay Selection:
There is a lot of choices when it comes to relays. I have seen everything from AC/DC open frame to sealed relay's being used for this on other websites. While they do work some present a problem at UHF. The Ameritron RCS-8V that I have on my other tower works well and there isn't much in the way of noticeable loss, but they are still open frame relays in an enclosure that isn't entirely watertight, so they do require maintenance if exposed to humid or wet weather. I didn't need a five position switch for the application and the RCS-8V is a little more then I wanted to spend. Their other models are recommended only for HF due to the relay's used.
I started going through the junk box here and realized I had an ample supply of a couple different types of T/R relays from Motorola radio equipment. One was a two piece relay that was commonly used in Micor stations. It has a coil that slides over the metal body of the relay, it's riveted and can not be opened, however, I have drilled out the rivet on these and replaced it with a self tapping screw if you get one with intermittent contacts. The output side of the relay is two RCA jacks. For the most part they are very reliable. I have seen these in Micor stations that were put into service as a repeater, the receive connection is pulled off the relay and the transmit continues to run through the relay. Consider the amount of on time the relay coil would see in this condition and how many times it would cycle on and off over the years with the repeater, if they can last thirty years in this application they will likely last more than long enough for me!
The second type I have is from Motorola trunk mount mobiles of the era, and still used in the slightly newer Syntor and Spectra series as well. The only disadvantage here is that you have to deal with the small cables coming out of the back of the relay versus just making your own with the other one. But you can get SO-239 shields with a small enough opening to use with this coax as well.
I had a good supply of the first type so I decided to use one of them. Leaving spare parts for future repair/replacement. You can find these on eBay or at hamfests for a few dollars. The ones I have collected cost me nothing, they were in boxes among other misc parts.
Enclosure:
Nothing expensive here, I browsed my local Lowe's and found a watertight pvc box and cover. It's sold as a junction box, it doesn't have any holes premade, and there is no knockouts either. The way these are used is the installer would drill a hole and install a conduit coupling to match what type your using, pvc, metal, flexible, etc. It's a good solid box for very little money, mine cost under $4!
Construction:
Now its time to get down to business!
My favorite tool for drilling the holes is a Unibit, some call it a step bit also. It makes a nice clean hole in both metal and plastic, I have even used it to drill a hole in plywood for making cable entry panels. When drilling the plastic go slow, and don't apply much pressure, just let the bit do the work. A quick tip if your drilling a metal panel with one of these, after the hole is the size you need use the bit from the opposite side of the hole you just drilled to remove any burs created around the opening, its a superb deburring tool.
I thought about cutting out an area of the PVC and overlaying an aluminum panel, giving me something metal to fasten the relay and connectors to. But it turned out that if I drilled a slight recess for the nut holding the relay that it would work just fine as well. I did this by allowing the next step in size on the Unibit to drill into the box until I had a deep enough shoulder for the nut and lock washer to sit against and be flush with the surface of the box. The other two SO-239's were constructed using RG-400 double shielded coax and some Amphenol panel mount connectors and shields that I found at a local hamfest. There is two machine screws and nuts holding each of them to the box. You will notice the hole on the right side of the box, this is for the control cable connection to the relay, it will be a watertight cable bushing, you can find them in the same isle as the box at most home improvement centers. That's up to the builder how you want to handle the relay power connection, you could panel mount a connector, drill a small hole in the bottom near the coax connectors, whatever. Off is position one, on will be position two. The relay is marked R and T on the casting, R is the normally closed position for receive and becomes position one, T is the normally open position for transmit and becomes position two.
I got my label maker and just made some quick stick on labels for the connections on the bottom of the box. You could just take a permanent marker and do the same thing if you don't have one.
Conclusion:
There you have it, a cheap two position remote antenna relay! I plan on powering mine through extra wires in the rotor cable from the shack to the tower. I have a Yaesu G-800SA rotor on this tower, it only requires four wires for operation, but I have the standard 8 wire rotor cable, two 18awg and six 22awg, so this leaves me four extra 22awg conductors out to the tower. Two of which will be used for the antenna relay, and two will become spares for future use. As things progress on the tower install photos of these connections will be shown on the Tower #2 page. But basically the rotor cable will come from the shack to the grounding enclosure on the tower, see Tower #1 for an example for now, and it will get connected to a ICE surge arrestor. From that point a smaller cable will branch off to the coax switch and the rotor cable will go up the tower to the rotor. This also provides protection for the remote coax switch power supply in the shack. As for control in the shack it will be a simple toggle switch, on for position two, off for position one. Doesn't get any easier than that!
Of course you could also build a power injector and turn the relay on and off that way as well, but normally most VHF/UHF weak signal hams that I have talked to do not power their preamps or switches by feeding dc-voltage up the coax. If they aren't going to do it than that's reason enough for me. According to those that I have spoken with it is because having a voltage on the coax can cause trouble in the connections over the long-term. It's just as simple to run some cheap wire out!
Back to Hints & Tips