[TowerTalk] Resonance of loaded Parasitic Elements

n4kg@juno.com n4kg@juno.com
Thu, 6 Sep 2001 08:33:24 -0600

N4KG response and further comments placed below previous

On Thu, 6 Sep 2001 08:51:08 EDT K7GCO@aol.com writes:
> In a message dated 9/6/01 4:04:19 AM Pacific Daylight Time, 
> n4kg@juno.com 
> writes:
> << 
>  A 5% lower resonant frequency for the reflector is a good place
>  to start.  It seems to me that one way to do this is to split and
>  insulate the reflector and then either grid dip the element or
>  measure the impedance at the center.  Once it is tuned, then
>  it may be joined and mounted to the boom.  Yes, there may be
>  some detuning effects but I would expect them to be small for
>  a 40M element.
>  73 / GL,  Tom  N4KG
>  On Wed, 5 Sep 2001  "K&L" <kmccourt@tbaytel.net> writes:
>  > 
>  > Hi All , I am in the process of building a loaded two element 40 
>  > beam. I
>  > have the driven element finished and its total length is 43` it 
> is 
>  > resonate
>  > @ 7.150. What would the most practical way to tune the reflector 
> be 
>  > ? Should
>  > I split & insulate the reflector , mount it on the boom and check 
>  > the
>  > resonate frequency ?  Is  5 %  longer a good length ? I am not 
> quite 
>  > sure of
>  > the best way method to use .  Would like to get some input on 
> this .
>  > 
>  > 73   Kevin
>  > VE3KKM
>   >>
> I've been able to grid dip aluminum elements without cutting them.  
> If you  have troubles getting a dip reorientate the coil.  Plan B is 
> to make  a shunt  link of 1 or more turns and connect it to the 
> element with hose  clamps.  
> k7gco.

I neglected to make two qualifying statements in my original post.

First, tune the reflector at least 1/4 WL above ground or
even better, at the final installation height.

Second, tune the reflector in the absense of any other elements.

Ken, K7GCO, makes a good point.  I forgot about that technique.
That is how I determined the resonance of my loaded boom dipoles.

I had an exact half scale model of my 3L20, namely a 3L10 on a
12 ft boom that was 1.5 inches in diameter.  I took a 2 ft piece of
wire, formed a single small loop that would fit over my Grid Dip Osc
coil, and clamped the ends of the wire to the boom, staddling the
center of the boom.  It measured close to 15MHz as determined
by a receiver tuned to the GDO.  As I recall, the measurement was
'tricky' with my Heathkit GDO but I was able to hold it steady long
enough to find the dip and then tune the receiver.  I *assumed*
that the boom of the 3L20 being exactly twice as big in length 
and diameters would then be resonant around 7.5 MHz.

Tom  N4KG

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