[Towertalk] Boom Matching TH7 on 40M

[n4kg@juno.com]

On Wed, 3 Apr 2002  w7am@juno.com writes:
> Hello OM's!
> 
> I ask your input, ideas and suggestions for feeding and matching a
> triband boom as a 40 meter rotary dipole.
> 
> Here, my 24 foot TH7DXX boom will be at 61 feet on a tubular, crank 
> up, tilt over tower.  So I do not have the option of tuning a matching
> network while atop the tower.  For me, modelling is not a viable 
> option either.

	How about a 'Cherry Picker' / Bucket Truck?
> 
> What could be done with the tower cranked down to 29 feet?  

	The feed impedance of a dipole is highly dependent
	on height above ground.  A full size dipole presents
	a feed impedance of 69 ohms at 0.2 WL (28 ft),
	91.6 ohms at 0.4 WL (56 ft), and 69 ohms at 0.5 WL,
	over perfect ground.   With modeling, you could
	find the impedance of a dipole at your desired height,
	and then look for a lower height that reproduces that
	same impedance and do your matching at that height.
	How much is it worth to get the answer?  N4KG


> A  temporary  1/2 wave feed line plus a temporary length 
> of ladder line with a  variable  capacitor at the bottom to 
> determine gamma capacity?
> 
	Be sure to MEASURE the velocity factor of your
	ladder line as there is considerable variation from
	manufacturer to manufacturer.   N4KG

> Preference is a slanted gamma rod of  RG-8 sheath with vinyl cover
> slanting from a boom attachment about 8 feet from the mast up to a
> waterproof box on the mast.
> 
	I recommend a gamma rod made of 3/4 inch OD
	aluminum tubing (or CATV Hardline if you have some)
	for compatibility of metals and weather resistance.
	N4KG

> Would there be any advantage dangling 2.3 foot wires from each end 
> of the boom to bring resonance up to 7mHZ?

	NO.  Why?  Where did you get 2.3 ft?   N4KG
> 
> I would very much like to hear what techniques others have used 
> together with gamma rod length and gamma or omega capacitor values.

	My TH6 rod is attached at the 10M director, whatever
	distance that is, and slopes to around 16-18? inches
	above the boom at the mast.  I believe my capacitors
	were in the 100 to 150 pf range but it's been a LONG
	time since I made them.   N4KG
> 
> DX forever!
> 
> 73,
>       Allen Robbins W7AM
>