Saturday, June 8, 2013

4M6M 7L68L4 antenna ready to launch

This is my latest workshop product.

After several evenings and this Saturday.
My new 4m/6m interlaced antenna is ready to put in the mast.

Propagation has been extreme poor last weeks some ES 1500km maximum.
WSPR rare decodes of VK 28MHz  and 80m absolute zero 800km DX maximum!
4m  wspr lot of decodes G0MGX 540km but 4m WSPR has still very low activity.

We had a few sunny days  still cool north winds but we could enjoy the first dinner outside in the garden without getting a pneumonia.
In the wind shield it was very nice  but this later Saturday afternoon the sky became grey and it cooled down with a nasty north wind to a cool 12C .
 I did cycle to the shop wearing a warm hat in June!   Strange cool weather low Sun activity I am worried this is not normal.

Despite the propagation meteorologic and economic depression I decided to make a bigger duo band Yagi/Una  designed by YU7EF  and  from the site of Janne OH5LID
It is a 7ele 6 eand 8 element 4m duoband  9m boom length. I already had the boom That was used before for  3 ele Fritzel the 9 ele VHF handbook 6m  then DK7ZB 7 ele then the LFA 7 ele.
And now I am going for the 4/6m interlaced  

I wanted to complete this project it today with help of PA1OX Jan.
Already had the boom and all elements fit together. 
Fine adjusting  the element positions took way more time
Hand made element clamps to keep it as light as possible gained  -40g  per clamp x 15 elements  but not easy to line up all parts.

Computer design output data  is not direct practical usable for construction especially when using tapered boom and elements. I had to convert it into practical usable dimensions.
EZNEC+ was used .
And the effect of using 10x1mm  tubes for 70MHz  instead of 12x1  is a bit strange . I would expect a thinner element should have a lower resonance frequency but it's the opposite.
Just a few mm for the 70MHz radiator but the farther directors  have to about 12mm more length .
Also the effect of the mounting plate and isolated or non isolated elements is not a simple straight forward compensation formula.
I would think if an element is fixed to an aluminium plate directly that part of the element behaves like a tapered section. Being thicker it would make the resonance frequency a tiny bit lower being only a relative short section relative to the wavelength  of 4meter.
Since a thinner element brings the resonance higher says the antenna optimiser program.
But no the element frequency is tuned higher by the thicker section.
Suddenly that part of the element is behaving different then a " normal"  thicker section?
 It is a complex thing ..    

Soldering aluminium elements can be done but to much heat will make weaken the ST50 aluminium alloy
So I replaced the soldered elements in the end by new ones.Should have been nice though.

Normally we should have nice June weather long daylight to 23pm but now it turned to a grey autumn evening.
Finally I decided to postpone the actual testing in the mast to tomorrow.
I am really curious to find out if it really works with matching and  so.
I want to compare it with my 7ele 3band    2ele 4 and 2el 6    that small antenna is doing really well.
The F/B ratio is surprising good must be 20dB or better not worse then bigger antenna's
 
 

some construction details for radiator and reflector Stauff clamps are used with M4 bolts and isolated 4x6mm parts of pneumatic propylene tubes filling up the gap between M6 and M4
Also the isolation is then better near the hole in those clamps where water can intrude in the gap between the clamp halves. This might be the reason that sometimes the excellent matching of the 3band  Yagi is showing higher SWR during rain when I think of this now.     Since I did not use this solution  then for M4 screws.
The connection to the radiator is done by clamping the blank wire to the element.
The 1:1 balun  or choke is simply done by placing ferrite rings over the short connection coax
I used 4 rings that is Xl about 4kOhm at 70MHz.
The same way I  use this with my 40-20-15-10m dipole. It perform very good.
At HF it resulted in lower RF level in the shack.









  


 
  






      

1 comment:

DaMorseMan said...

Waar koop je die Stauff clamps ?

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