[OZAPRS] Newb HF questions ;)

[Ray Wells]

Hi Terry and all,

Consider that the impedance of a quarter wave ground plane (with 
horizontal radials) is about 35 ohms. Mobile radio manufacturer Pye used 
to use 35 ohm coax in mobile VHF installations many, many years ago, 
back in the days of their all valve Reporter series AM mobiles.

If one "droops" the radials downwards the antenna starts to more and 
more resemble a centre-fed dipole, and the impedance rises accordingly. 
About 45° increases Zo to about 50 ohms. Another (less convenient) 
"trick" is to increase the antenna length, and a value of 5/16 
wavelength returns a value of 50 ohms (give or take, considering length 
to diameter ratio - fatter antennas have lower Zo). However, a 5/16 
wavelength vertical also shows inductive reactance which needs to be 
cancelled out and this is easily done with series C. The 8.5m antenna 
when used on 30m is somewhere between a 1/4 wavelength and 5/16 
wavelength, hence my "guess" at a Zo of about 40 ohms.

Let me throw more confusion at you :-) Re the 4:1 balun, is it stepping 
up, or is it stepping down? A quarter of 50 is 12.5 ohms which would 
provide a "better match" for a typical (very) short HF whip. The balun, 
of course, doesn't provide any inductive reactance to bring the antenna 
to resonance. One antenna manufacturer some years ago made a 2:1 
transformer that went into the base of their HF whips to provide a 
better match to the coax. I "think" it was scalar, in the 1970's - yep, 
it's in their 1985 catalog.

Ray vk2tv



Terry Neumann wrote:
> Damien Gardner Jnr wrote:
>
>> Hey Norm
>>
>> I'm a little confused by the balun too ;)
>>
>> 50 ohms u/b from the rig to the atu
>> atu is u/b (50 ohm?) to the PL259 on the bottom of the balun.
>> balun balanced side has one terminal to the bottom of the TEV-1, and  
>> one side to the 'radials' terminal on the mounting plate for the  
>> TEV-1, which is bolted directly to the mast.  I can see why you'd 
>> have  the 4:1 balun if you were feeding a dipole, but as ray said 
>> earlier,  on 30m, the TEV-1 is pretty damned close to 50 ohms, so I 
>> would have  thought putting the balun in there would completely screw 
>> the vswr??   (it'd be giving me ~11 ohm on 30m yeah?)
>
> It certainly should, and would most probably produce problems similar 
> to those you are seeing.
> Take the balun out as a first step.   The presence of the balun on the 
> TET Emtron web site is a real puzzle.  It could be useful at some out 
> of the ball park frequency, but flies in the face of everything one 
> reads about the feeding of vertical antennas elsewhere.     If you use 
> the balun at all, put it right at the transceiver /atu and run a 
> balanced line  (300 ohm ribbon?)  from there to the feed point of the 
> antenna and the ground system at that point.
>
> The TEV-1 at a length of 8.5 metres  calculates out as a quarter wave 
> vertical at approx. 8.8 Mhz.   This is a very long way from any 
> amateur allocation and as such  in a "ideal" ground plane antenna 
> would exhibit a feed impedance of 50 ohms only at that frequency 
> (8.8Mhz).    Something is very seriously wrong if it "is pretty damned 
> close to 50 ohms" on 30 metres as is.  Something else in the system is 
> doing the most of the radiating - or absorbing the RF energy as heat 
> somewhere - or both.
>
> 73
> Terry
>
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