2m Slim Jim Update

It's some years now since I first put together my copper tube Slim Jim for 2m FM working.  If you are consulting this blog to find the right dimensions, this site is reliable - except the feedpoint may well be at a different position, which you will find by trial-and-error.

The Slim Jim antenna, made for pocket money, has proved to be a very good performer indeed. Contacts over distances in excess of 90 miles are fairly common with just 20W, and out to beyond 180 miles when conditions are slightly enhanced.

Recently, not satisfied with a temporary-that-became-permanent, too low mounting position for some radiating directions, I transferred the Slim Jim to a loftier height on the roof.  This has led to considerable improvement in longer-range working.

Many of us have one of these generic, cheap SWR meters.  They are accurate, if used with some thought.

My VHF SWR meter, which is one of those cheap-and-cheerful, but apparently accurate types from China, reported a good match.  Yet, the power out was about 5W lower than my new Yaesu FM rig ought to be putting out.  Leaving the meter on the low range whilst sending about 10W out also permitted the reflected needle to climb to ~2:1.

Something was clearly out somewhere.

So, armed with my trusty SARK-110 and some very calm, clear conditions at night, I climbed the ladder to pull ol' Slim down, and see what was going on.

The refurbished Slim Jim, sitting high - and with a low SWR.

I stripped the self-amalgamating and PVC tape wraps off the feed points, and un-soldered them from the antenna.  Using pipe/Jubilee clips, I sought to find the resonant point with the antenna on a temporary pipe clip mount on a shed.

Having found a perfect match at 1.1:1, I re-soldered everything and wrapped up with tape - only to find that the SARK now reported 3:1 SWR!  After a second or two of WTF? I realised there must be something amiss at the cable.

I'd left short tails on the antenna that were meant to 'take the heat' from soldering, avoiding damaging the feedline cable.  These connected via spade connectors.  One of them had either a dry joint or had broken somehow.

I also found that the gap I had on my Slim Jim was too big.  I followed some online design which was in agreement with others in terms of length.  But the gap was about 4 inches, whereas it needs to be an inch, perhaps slightly less.

The way I adjust for minor inaccuracies like this is to solder a bare copper wire that's thick enough to be dimensionally stable, yet bendable by hand.  Craft wire is often ideal for this.  Wrap the joint in self-amalgamating and PVC tape to keep water out, and then just bring an 'L'-shaped arm down to a point where the SWR lowers to perfection.

A wider-than-ideal gap is adjusted simply by a stiff, bare copper wire, soldered to the top pipe.

The SARK finally reported a 1.09:1 SWR at 146MHz, and an impedance around 53 +j4.9Ohms.  The Chinese analogue SWR meter now also agreed, showing the correct rated power outputs when keyed. It's a remarkably broadband antenna, yielding well below 1.5:1 SWR over a span greater than 6MHz!

Near-perfect matching across the band.

Another 30 minute task that took 3 hours, but with a productive outcome.