[TowerTalk] Hair Pin feed - ???

[Eric Gustafson Courtesy Account]

>From: "Robert W5AJ" <w5robert@blkbox.com>
>Date: Sat, 4 Jul 1998 14:51:13 -0500
>
>Have my new MFJ-259B in hand.  Have a 5 element six meter yagi
>with hair pin feed.  Have shortened both driven element and hair
>pin down from spec's and have:
>
>1.2 at 50.400mhz & R=43
>1.5 at 50.125mhz & R=35
>1   at 48.550mhz & R=55
>
>The R value takes a dip right where I want to operate.  How much
>of a hair pin movement vs. resonant point?  I can calculate what
>1/2" change in driven element length will move the resonant
>point but have no data for hair pin.  comments?  info?
>
>73  Robert W5AJ
>

Robert,

W4RNL has some good information for you as to doing the
calculations.  

However, from the data you posted, it looks to me like you are
probably already as close as you are likely to get with
calculations or modeling.

Here is a way to proceed to get the system tuned to 50+j0 where
you want it.  You may have to iterate this process a few times to
get exactly on.

1.  Make sure that the feedline shield is decoupled from antenna
    currents.  This can be acomplished in several ways and may
    already be accounted for in the antenna design.  Just don't
    start tuning until you are certain that the feedline shield
    is RF "cold".

2.  Adjust the hairpin length so that the resistance reads 50
    ohms when the reactance reads zero.  Don't worry about what
    frequency this happens to be at this point.  The apparent
    resonant point will be changing with hairpin adjustment.

3.  Adjust the driven element length to move the system resonant
    point (zero reactance) to the desired frequency.

4.  Repeat 2 and 3 above until the system is close enough to your
    goals to make you happy.  Be sure that when you stop, step 3
    above was the last operation.

You give no reactance values so I can't help much with the
initial direction to start moving the hairpin.  But you can find
this out in short order by cut and try methods.

Hope this helps!

73, Eric  N7CL

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