New Antennamabob!

Well,

For quite some time, I have thought that I would like to have gain and front to back on the 12M and 17M bands. Currently my MA5B is only a rotatable dipole on these band. The MA5B is an excellent antenna and overall I have been delighted with its performance.

You may recall I tried this antenna for a while and found it to be a load of dingos kidneys.

So, Cushcraft (or MFJ as they are now) have introduced a new variant of the MA5B called (yes you guessed it) the MA6B. It adds a reflector for the 12M and 17M bands plus an additional driven element and reflector for 6M.

So I have purchased on from my ever helpful chums at LAM Communications.

Now, here's where the fun started. The first problem I noticed was that the main boom tube insert was missing completely from the pack; I had three of part number MA6B-BC when there should only be two and part number MA6B-BE was missing completely. This made this stage of the manual challenging:

The part I have circled above was missing. So, rather than wait for a replacement to get sent and damaged in the post, I headed to a local metal stockist and get a longer tube of the same outside dimensions for GBP 24. Now, holding these four bits of metal together with only worm clamps (in the UK we call these Jubilee Clips) in my opinion is not going to hold. The leverage of the elements is going to make this spin in windy conditions, so I have modified mine either side of the junctions with some bolts through the boom to stop any spinning:

So, I then noticed whilst constructing one of the elements that the tube wouldn't fit together, here is the end of one of the components:

Hopefully you can see that it's kind of swagged at the end and didn't fit into the slightly larger pipe it is meant to slot into. So some filing was in order.

I found that the 6M reflector insulator plate as shown in the diagram below was missing, I had to make something from some sheet PVC I had here:

I also found great confusion over the dimensions of the 12M and 17M driven element and reflector. Now this is where it gets complicated. This is the dimensions for the driven element from the manual:

So if we first look at the dimension from the centre of the boom to the trap, this is listed above as being 74.5". Now, the individual parts are:

2.25" from the centre to the start of the metal - this is the exposed part of the insulator.
Part ED is 46" long
Part EB is 36" long but inserted 5" into ED so we have 31" in length

So that's 2.25 + 46 + 31 = 79.25" which is definitely NOT the 74.5" as above.

The trap is 11" and the end piece is 12" with some adjustment so we will call that 13".

So the element is 79.25" + 11" + 13" = 103.25" - the manual says 98.5". I wonder.

The reflector dimensions are:

Now, this one is better, the individual parts are:

Half the reflector centre piece (EF) is 42"
the element insert is 41.5" in the manual but is actually 47.5" in reality - it says to insert 5" (I think) but it doesn't actually say in the manual - that bit seems to have been deleted in a recent edit. So lets call that 42.5"
The trap is 11"
The end is supplied as 7" but MFJ have already said thats an error and the part should be 13.5" with some adjustment so we will call that 14.5" for now.

Note that I have manufactured my own parts for the end piece EJ and they are 13.5" as stated in the on-line manual. The paper manual that came with mine says 7" and the supplied part was 7".

So we have 42" + 42.5" + 11" + 14.5" = 110" - the manual says 102.5"

Now, I've built it as described by the individual element construction instructions, and here are the SWR curves, starting on 20M:

17M:

15M:

12M:

10M:

So it looks very much, like the 12M and 17M elements are too long - so perhaps the manual is correct in the overall dimensions but not the individual element instruction?

The other fairly odd thing is that the 17M bit of the antenna is capacitance loaded on the reflector but not the driven element - so you can't really compare the two against each other.

If I look at the instructions for the OLD MA5B antenna, the 12M/17M driven element dimensions are centre to trap 79.25 then 11" trap plus 12" end - so the same as the driven element in this one when built as per the element construction - not as described on the "element length" diagram.

So I suspect the first thing to try is to build the antenna to the "element length" diagram and see what happens. This will mean inserts of 9.5" on the driven element and a stunning 12.5 on the reflector instead of 5" on both as per the manual!

I just wish MFJ would tell me how long the elements are supposed to be! I opened a ticket with them on 20th August; they have offered to send me the correct 13.5" parts, but have not commented on the lengths of the elements that are wrong in the manual:

So I'm not sure which way to go now.

More to follow!

** UPDATE 1 **

I have set the antenna dimensions to the overall measurements on the diagram "element length diagram" and ignored the previous dimensions for how far to insert the tubes together. So the antenna is configured like this: