Slim Jim Antennas and the Fixing Problem Solved.

For a few years, I've enjoyed good service from my homebrew 15mm copper pipe SlimJim.

With a compressed, low angle pattern of radiation, the SlimJim, whilst hardly a 'supergainer' antenna, does put the gain it has in an ideal place for operating repeaters and point-to-point FM.

My current fixing method is a 4 metre piece of timber with a few PVC pipe clips holding it fast against the violent, 85+mph winds we get here on a regular basis.

The existing SlimJim fixing arrangement.  Strong, but it's too low.

Over the past weeks, it's become clear that my fairly low position for the antenna, whilst fine in almost all directions due to our high ridge position, is causing a loss of signal towards the southern parts of north Wales.

So, it's been time to consider moving the SlimJim up a little.  As luck would have it, I have some spare capacity on an old satellite TV mast, which is fixed on super-heavy duty double brackets that will take any wind thrown at it.

Attaching a SlimJim to a metal mast is more problematic than you might think.  It really needs to be isolated from the mast, so some insulating arrangement is required.  That said, I have attached a SlimJim directly to a metal mast, which seems to have little effect, other than an increase of about 0.1 in SWR from 1:1.2 to 1:1.3 - which you can cancel by adjusting the feedpoint position a little. 

I've come up with two solutions that both hold up to strong winds with this very top-heavy antenna.

First, you can take a small piece of aluminium plate and attach Stauff clamps to them, such that these firmly hold the bottom of the antenna.  You then use a couple of the usual 'U' bolts (stainless, to avoid electrochemical problems), to fix the bracket to your pole.

A fine bracket solution by G0SXC, selling on Ebay for very good prices.

The second method is more rough-and-ready, but it is strong.  You use a 15mm to 22mm copper coupler, which you solder onto the small tail at the bottom of the SlimJim.  Simultaneously, you solder a shortish length (maybe 6 inches) of 28mm copper tube onto the coupler.  

The pipe at right is 15mm - a 'simulated' lower tail of the SlimJim - fitting into a 15-22mm coupler, soldered to a 28mm pipe.  PVC tube of about 21 or 22mm (or slightly larger) fits over the tube.  You need two split lengths to make a correct, tight fit inside a 1.25" mast pipe.

 
Whilst this is cooling, take a similar length to your 28mm copper pipe of 21/22mm PVC pipe, and slit it lengthwise with a rotary cutter.  You then slip this over your 28mm pipe, forming a sleeve.  It's a tight fit! Take another slit length of PVC tube and slip that over the first sleeve.  This is an even tighter fit!

With all this in place, you'll find that it will fit very snugly indeed inside a 1.25" aluminium or steel mast tube.  You can use a small collar made of tape or a hose clamp to stop any downsliding, or fit a stainless bolt through the mast tube where the inner tube can rest on it (you will need to make sure there is a length of PVC resting on the bolt, and not any copper tube, to maintain the electrical insulation.)