Topband: Thoughts on making low resistance joints between aluminiumtubing sections?

[Tom Rauch]

> I am very much aware that any joins between sections of a 
> short vertical
> radiator need particular care to produce as low a 
> resistance as is possible
> so as not to degrade the overall efficiency any further.

In my experience that's largely a needless concern.

We used to do that on broadcast towers where the sections 
were flanged and paint might interrupt the connection, but 
even then the bolts used to crush right through the paint 
and make a connection. Older installations often included 
jumpers around joints, but that proved through exoperience 
to be a needless and unwarranted effort.

You put thousands of pounds per square inch between two 
metallic surfaces and anything there moves out of the way.

For example I put a 40 meter aluminum tubing vertical up the 
other day and didn't have a hose clamp handy to clamp down 
the last section where it attaches to the base. I just slid 
the tubing over the metal post from the base and let the 
weight push down and it rested on a round rivet. While I 
will eventually clamp it, the pressure from weight alone is 
enough it doesn't cause any problem at all.

My yagis don't have any special provisions for connections, 
they are perfectly fine. The Force 12 Yagis use what I 
consider a horrible arrangement of tubing inside tubing with 
only a pop rivet holding the joints. No heating, no loss, no 
noise on receive. My other yagis use compression clamps 
and/or screws. No problems ever. My M2 six meter yagis that 
I never thought would never hold up use a single #8 
stainless screw through the elements at the very highest 
current point, the center of the elements. The elements just 
lay in a cast and machined aluminum cradle held only by a 
single screw compressing on the element. After years in the 
weather no problems at all. No rectification, no 
non-linearity and no arcing or signs of heating (I just had 
one of the antennas down and took it apart to look) despite 
almost 1500 watts of six meter CW power and the connection 
at the worse possible area.

I've seen at least a hunderd AM BC towers without joint 
jumpers and never have found a case ever where the jumper 
was necessary.

> How do people tackle this problem (and it would include 
> Yagi constructors)
> of making low loss contacts at each section where an 
> aluminium tube is
> fitted into the previous section.

I use a grease so I can take the element apart later. Other 
than that, I don't worry a bit about it.

> Are there better ways than say using sandpaper and lots of 
> elbow grease to
> make bright clean joins which are then covered in good 
> conductive grease?

Stick your ohm meter leads in that conductive grease. Tell 
me how conductive it is.

The grease stops oxidation and helps you get the thing back 
apart someday, but you'll never develop a lossy connection 
at a clamped or bolted joint. If you do, it will arc over 
and heal the first time you transmit. My Force 12 antennas 
have the worse possible connections through pop rivets, and 
they aren't a problem. I can't imagine a clamped or bolted 
joint being an issue.

> My problem is exacerbated because I am not using new 
> material but rather
> stuff that has been pre-loved.

Polish it up by wet sanding and rinsing, slap a little 
Noalox grease on it,  and just clamp it or bolt it. Anything 
else is a wasted effort and causes more problems than it 
cures.

The force in pounds per square inch of even a single #6 or 
#8 stainless screw is so high you'll never have a problem.

73 Tom