Given some of the activity that I’m seeing on 10m, last night I went to build a quick and dirty 10m vertical dipole but found I was lacking a couple of the bits that I wanted, so given that I was going to ave to buy some things anyway, I decided to make something a little better.
A little web searching showed up this article. The only thing that I was unclear on is where the author got his percentage of wavelength sizes for the driven element and the coax from. I am assuming it’s an adjust meant for velocity factor but he used Aircell 5 which has a velocity factor of 88, in which case the length of the coaxial element is substantially short as it looks to assume a velocity factor of 60%. I can’t find any coax with that velocity factor.
I’m assuming that it is velocity factor. I will be using RG-58CU the vf is 66, and I will assume 98 for the aluminium pipe that I’ll use for the driven section, the measurements that I will run with for a vertical centred at 28.300 MHz will be
Coax element – 1.749m
Driven element – 2.597m
The basic Idea that I have is this:
I’ll actually cut the tubing a little shorter as I also bought a short length of 10mm tubing that I can put inside it, for fine tuning. I’ll use a hose clamp on it once I have the length correct.
For the choke, I’ll do about 12 turns of RG174 around a pair of ferrite toroids. This will be mounted in a small box. The antenna coax will connect to the choke via BNC connectors and the antenna via an SO239.
Well, there is the theory. I guess I need to start looking at some practice now.
Step 1 – The Choke
Similarly to the linked article, I decided to use a pair of 35mm Ferrite toroids, but as I couldn’t quickly lay my hands on some RG400, I made it with 12 turns of RG174U.
I was sure that I picked up some round lugs today so I could do the earth of the SO-239. I must have dropped them before I got to the counter. What I ended up with is a little less tidy, but it works. I also put a 25mm M6 Nylon bolt through the case to old the toroid in place.
Checked it for shorts, then for completeness had a look at the inductance of the braid. 215.1 µH.
and there we have the completed choke.
Next step into get the BNC on to the coax end.
With a bit of heat shrink for tidiness and good measure.
and it attaches properly to the choke. The next part will be measuring and cutting the coax.
That’s the coax cut to 180cm with the braid cut at 175cm, which hopefully should be the values I need.
Now comes some work with the hardware.
I can’t find my tape measure to work out how much to cut off the 3m tube, but I recalled that the bit of wood I bought was just over 2.2m. Given that I have a 1m adjustable section that I can slot in, I decided to cut the 12mm tube to that length.
Next step is the U-bolts into the bit of decking that I am using for the bottom.
I’ve placed the U-bolts about 25cm apart and I’m using the 80cm section of the 12mm tube to make sure that I’ve got sufficient washers to bolt it up relatively tightly.
That’s starting to look pretty good.
Now, given that I forgot the round lugs, I need to reconsider how I attach the centre conductor of the coax to the tube.
It occurs to me that I bought two hose clamps. I could clamp a spade connector to the tube with that.
It’s coming together and I’m starting to get a little excited about being able to test it tonight.
OK, it’s not properly visible here but there is a minor problem. That 5cm of centre conductor that I have connecting to the bottom of the tube means that the choke actually sits below the end of the pole. I’ve either got to move the pole up a little, which means moving the bottom U-bolt, or cut some of that centre conductor off, which is likely what I will do.
However, I’ve got a bigger problem.
I wasn’t paying attention when I put the 10mm tube inside the top of the 12mm tube. I’ve got the end tat I sawed at the top. Burrs and all. The 10mm tube is not as far down as I need it, and it’s now stuck. Nothing I can do will get it out. I’ve made a right mess trying to use multi-grips to move it. I suspect the best thing to do is to get another couple of bits of tube 😦
Given that I was resigned to buy more tubing, I decided to cut the 10mm tube off as I still had sufficient to put in the other end.
So I’ve turned the 12mm tube upside down and you can see the nice mess I made trying to get the 10mm tube out. I’ve also still not shortened that 5cm length of coax inner conductor.
As you can see I still couldn’t get the 10mm segment in quite as far as I wanted.
This is the mess from the end that I cut out that is now at the top. I bought a rubber foot to put on the top but not quite sure what I’ve done with it just now.
Anyway, I leaned it up against the house and fired up the antenna analyser.
It looks resonant (SWR 1.01:1) around 27.300 MHz, so still a bit long I may need to cut off some of the top.
However it’s still giving better than 1.5:1 on 28.490, so I’ll be giving it a work out tomorrow when things open up a bit.
An interesting aside. I’m also seeing around 1.2:1 on 12m!