I have decided to do away with my remote matching unit for one in the shack. I originally used a remote unit because of the difficulty in feeding the antenna from the shack.

What I have done is to run 300Ω twin feeder inside 20mm conduit over the top of the side entrance to the bungalow, all the way to the feed point. It's about 26m long and the conduit has no discernible affect on the feeder, I do know that plastic conduit is a very good RF insulator have previously tested it, it also keeps the feeder dry, which is handy if it pours with rain.

The matching unit I have built and it is a double PI network, this is a PDF of the circuit:

https://www.dropbox.com/s/ntvwoc9tk7pzr9j/Double%20PI%20match.pdf?dl=0

I made a wooden box rather than metal because I wanted to make it as compact as possible for my small shack, it has turned out not much bigger than the K3, just slightly wider at 300mm and the same depth as the K3 250mm and not quite as high as the K3 at 90mm, these are the pictures of the box:

https://www.dropbox.com/s/zj7od0rd59tsk5x/IMG_1393.JPG?dl=0

It doesn't look very pretty at the moment because it is work in progress. I have used 4mm Banana plugs and sockets for the RF switching to save on the complexity of switches and to keep the runs short.
This is the inside of the box.

https://www.dropbox.com/s/6vnspetmdm8sq0l/IMG_1394.JPG?dl=0

The coils are home made. There are two additional capacitors with flying leads terminating in banana plugs that I can place across either the TX side which is 1500pf and and additional 250pf across the antenna side if needed, this is really to save switches again as the capacitors are only used on a couple of bands.

A few comments on what I have found so far, having analysed the feeder plus antenna at the shack end, the losses on the open wire line are no worst than 1.3db, strangely that is on 60m where the standing wave is pretty high on that band and the lowest is 10m where it's 0.43db because the match is pretty close to 300Ω with a reactance of only a few ohms.

It operates from 160 through to 10m but because the antenna is short on 160m (30m total) it isn't very good, not only that the standing wave on the 300Ω feeder is something ridiculous like 1000:1, but the matching unit will match it. But I strap the feeders on that band and use the antenna as a Marconi Tee and feed it with the SGC-230, that works well.

I notice that the tuning on all bands is quite broad, none of this sharp tuning like you get on T match networks and only one matching setting, not like T networks where you can get two and sometimes more settings.

If I want to use it as an unbalanced AMU, all I have to do is short out one side of the network and replace the Balun with a coaxial choke and it will feed and end fed antenna, versatile

I hope this might spur some to think about trying their hand at building one of these units, they are very simple to make and they seem to do the job nicely.

Nice little project. A friend has just made a lovely double Pi tuner.. very nice.

What are the inductors between the capacitor chassis and the balun output? I'm assuming they're inductors. Can't see them on the schematic.


I really need to pull my finger out and make my s-match. :(

They are 10MΩ static bleed resistors, I normally put them at the bottom of the feeders in the garden into an earth stake, I might still do that. One leg of the antenna is grounded through the Balun, but the other side isn't, not knowing which is which I put resistors on both feeders. I've always done that having coped a packet off the feeders at my old QTH when a storm approached.

As you know I have built the S-match, but I do find the settings on some bands to be quite sharp, that's why I went for the PI match, they are much broader.

By the way, the large capacitor has both the variable and fixed vanes isolated above earth, it's a bit of a rare beast, I've not seen one like that before, I got it from the Dunstable Downs radio rally many years ago.

Ahhh.... A wise precaution. They look quite beefy... what power rating?

Yeah... the double pi will be broader, but the s-match will be easier to make a bomb proof, high power ATU that will almost certainly outlive me ... I have a capacitor here that looks like it was forged in the fires of Mount Doom that needs putting to good use

The resistors are 10MΩ at 1W and are 5kV AC, RS part number 683-5326, I hope I never reach that sort of voltage with 100W

Don't forget to post some pictures of it when you've built it, it's always nice to see what others do.

Oh yes... you'll get photos

Paul

Is the conduit with the feeder in it fixed along a wall? What colour is the conduit? I've read in the past about black conduit with it's carbon content effecting things.

Stewart

Hello Stewart,

The conduit is black and no it doesn't contain anything that affects RF, try placing a sample in a microwave oven for a couple of minutes with a cup of water next to it to act as a load, the plastic should remain cold, if it can take 800W of 2450MHz I'm sure it will take 30MHz without a problem. As you say, some plastics do contain carbon the results are quite interesting also some nylon loaded plastics are poor RF insulators, the microwave soon sorts the men out from the boys

It's only attached to the wall for about 4 metres, I did try spacing that distance off the wall by about 40mm without the conduit but it didn't make any difference to my measurements when I put it inside the conduit and spaced it off the wall by the distance of the saddles, which is something like 3mm.

By the way, it has got some bends in it, three 90º bends, what I call sweeping bends using a bending spring, so the are not sharp, being an ex-sparks I'm expert at using that, it takes a bit of practice and one sharp 90º bend, I'm not sure what affect that has, but it doesn't seem to make much difference.

Lastly, in my situation the feed point is amongst all the shrubs and an evergreen tree, that made it easy to get the feeder to where I needed it, just push the conduit through the vegetation and let the branches support it then just push the feeder though the conduit, the conduit keeps the feeder from touching the branches.

Thanks Paul. I'm going to give the conduit a go. I've only got the one bend, Ill have to find my bending spring. Mine is going to be fixed along a wooded fence, about 6m long.

One last question, where did you get your feeder from?

I got the feeder from Westlake Electronics.