TopBand: folded unipoles

Wed, 8 Oct 1997 09:02:07 +0000

> From:          Peter Chadwick <Peter.Chadwick@gpsemi.com>

Hi Peter,

> If I have a vertical with a feedpoint resistance of 10 ohms, and one
> with a feedpoint resistance of 100ohms, the ratio of the currents will
> be square root 10. An inductor with a series resistance of 1 ohm will
> dissipate 10 times the amount of power in the 10 ohm case. So  I squared
> R losses are reduced by using a higher feedpoint impedance.

If your hypothetical antenna is 10 ohms j0 compared to 100 ohms j0, 
the matching inductance required in a conventional L network on 160 
is 1.77 uH for a ten ohm antenna, and 4.42 uH for a 100 ohm antenna. 
Those are inductive reactances of 20 ohms and 50 ohms respectively.

With 1500 watts in a conventional low pass L we'd have about 12.3  
amperes of current in the inductor, or a loss of 150 watts using your 
one ohm figures. But I think it is rather unfair to use an inductor 
with a self Q of 20. Even a moderately well designed inductor has a Q 
of  200 plus, with Q's of 400 being practical with lots of work.

With a very practical coil Q of 200 (easily obtainable with tinned #8 
wire), that would be a coil series loss resistance of .1 ohm for the 
ten ohm system. This is now a power loss of  about 15 watts 
in the inductor! 

If we eliminated ALL the inductor losses we would have an 
additional .05 dB advantage. " No sale " on that theory, unless your 
using a nichrome coil.

> To see matching components catch fire, just look at 500KHz 1KW marine
> tx's running into 30 foot whips

This has nothing to do with the topic of folded unipoles. In this 
case the problem is VAR power due to the highly reactive load. A 30 
foot whip on 500 KHz is like a eight foot whip on 160. Fold it up all 
you like, and chant all the magical incantations about "radiation 
resistance" you like, and the nasty voltages and attendant losses 
will all still remain.

> transmatches - call them what you will - commercially available suffer
> from excess Q when matching large impedance ratios, and in mnay cases,
> this is evidenced by tank coil heating. There was a review in QST
> recently of ATU's that evidenced this.

(But keep in mind the loss data published in QST was incorrect, the 
measurement method was too sensitive to produce reliable results 
PLUS Frank Witt made a calibration error. Of course even though they 
admit that privately, don't hold your breath for them to admit they 
made a mistake.)

But once again, this has nothing to do with folded unipoles. The 
problem is AGAIN VAR power, because most tuners lack enough C for 160 
causing the VAR power to be very high.  Once again it isn't a problem 
of the RESISTANCE, it is a problem of the REACTANCE in the system. 
The small C's in the tuner cause too much reactance, and too much VAR 
power.

With that in mind, folding the antenna would not change the tuner 
loss any noticeable amount.

> Taking the definition
> " The total power radiated as EM energy divided by the square of 
> effective current causing that radiation taken at the current maximum 
> of the radiator."
> 
> In a folded unipole, where is the current maximum, and what is the
> effective current?

ALL common mode current at the current maxima is the effective 
current, just like in the folded dipole.  The current maxima, in a 
1/4 wl or shorter vertical marconi antenna, is at the base. 

> I guess we all accept that the feed impedance has
> gone up, and the feed amps gone down.

The FEEDLINE connection point amps have gone down. The effective 
radiator amps, and hence the ground system amps and loss, remain the 
same.

> Does this not mean that it doesn't
> radiate as well as a non folded unipole if the radiation resistance is
> the same as the non folded unipole?

It radiates exactly the same. Same same same. You have the same 
net radiator current (divided between two close spaced conductors), 
the same ground loss, the same everything EXCEPT what the feedline 
"sees".  You might as well stick a 2:1 transformer at the antenna, 
and  claim you have improved things that way. The results are the 
same.

Of course if we want to "nit-pick", and count the angels standing 
on the head of a pin, the THINNER the radiating structure has HIGHER 
"radiation resistance"! That's exactly the opposite of what is 
claimed, but generally the difference is so small no one would ever 
notice. 

> Plus the fact that it indubitably works!

It should work, it is the same thing as any other vertical the same 
height with the same ground. Even my ten foot mobile antenna "works" 
enough to allow QSO's into Europe, South America, and VK on 160..  
and it is only about 1% efficient.

73, Tom W8JI
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