Back to the Radio Pages

Here is series of photos that attempt to detail how I reworked the driven element of my M2 6M2WL yagi on 50 Mhz in 2022.
While the SWR measures ok, the real problem is you slowly get a DEAF receiver over time with the original M2 feed block.

For the past 5 years I have been dabbling in 50 Mhz EME and during the whole time my receive system has always been deaf no matter what I tried.

About 2 years ago I refurbished a second hand M2 6M2WL to be used as a dedicated EME yagi.  This for me involved an upgrade to the driven element, bringing it up the the M2 6M2WL-C spec, which seemed to be a natural choice to use the latest feed design for that yagi.

It turns out this was a major bad decision.

The freshly refurbished yagi performed beyond my wildest expectations and while it is mounted on a short tower just 8 metres off the ground it was easily 10 dB better than its twin brother on a 50' tower at moon rise and at elevation.

Woo Hoo I have fixed my RX problems or so I thought

 Over the next year the refurbished yagi quickly deteriorated to be worse than its twin brother on the tower and sent me down a rabbit hole of investigations into WHY...   water in the coax?  bad length of Heliax? did I put it togther wrongly?  Did the horse nibble on the reflector?  I was loosing sleep over this.

It turns out the matching bars have a joint where Copper and Aluminum are in direct contact. Arrgh. The weight of evidence as to why this is a bad idea is well documented in any number of books and web sites beyond number.


orignal feed block
This is the original M2 feed block, well I say original but there was some rescue work done as this feed block had one 'leg' cut down to just a stub of its former glory as it was brought back from the USA in aircraft luggage. You will be able to see the copper tabs that pole out of the two matching bars (3/8" rods)  The black cylinder is a sleeve of 'Delron' which is the same material as M2 element insulators that keep the matching rods in place when assembled.

oval holes
Close up of the matching block, looking in the end where the Delron insulator was removed. it shows the centre pin of the 'F' connector and the link wire to the main N connector (this needs a tidy up before final assembly)

The matching bars appear to be created by the insertion of a strip of copper into a slot in the some aluminum rod, this rod is put inside the 3/8" tube and crimped closed to provide a connection that can be soldered too.  While it kind of looks cool and for some period of time works okay, over time the miss match of having copper and Aluminum in direct contact takes its toll and it all goes to poop. 

The company themselves have a web page dedicated to the problem but without properly identifying the problem head-on they offer to refurbish you existing feed block at your cost 

So how do I go about rebuilding the feeds on the two M2 yagis I have, I had considered sending them to scap and starting again but we are in the middle of the COVID 19 pandemic and it makes things complicated with lockdowns and big transport problems.

The obvious answer seemed to be avoid a copper / aluminum connection but how....   Stainless steel seemed to be the obvious answer as it does not have a problem at the connection point with Aluminum or Copper so it could be my middle-man for the connection.  Now do you realize you can SOLDER to Stainless steel if you are crafty?

how bad was the hole?
Here is a new length of 3/8" tube destined to be one of two new matching bars for the yagi.  They have a M8 thread cut into them, that screw sticking out, was a M8 pan head philips screw that a friend has turned down in his lathe to be the same diameter as the 3/8th tube. 
The M8 thread has been only cut deep enough to ensure that the last two turns of the screw start to seriously bind up.

original matching bar
This one of the original matching bars, you can see the copper TAB sticking out. the 'gunk' you can see is remnants of the silicon goo they fill the matching blocks with the keep moisture out of the system
copper to stainless interface
Here we go, this is the magic, copper wire soldered to an Stainless bolt.  The secret is the use of 'rust converter' as a flux, its the Phosphoric acid that does the hard work and allows the solder to bind to the surface
new matching bars
Here are the parts for my re-worked M2 6M2WLC feed ready for a test fit, I have not yet fully committed to screwing home the stainless interface screws yet as its just a test fit, anything could go wrong yet : -)
rods in, wow it fits
Wow rods fit and I can get to all the parts I will need to solder, next step is to trim the wires and commit to a final assembly, wish me luck. find me here: pedroj at internode dot on dot net