[TowerTalk] Rising Input Z with more radials?

[hasan schiers]

Tom, you were correct about the rising Z. Thanks so much.

I found out why the input Z was going up as I put more radials in...I was 
measuring at the end of 55' of LMR-400 (as opposed to the measurements I 
took when I was outside originally tuning the antenna.), and that was acting 
as a 1/4 wave transformer.

I just went outside and connected through an 18 inch jumper to the MFJ-269 
and got the following measurements:

2:1 VSWR Low freq Point: 3460 khz
2:1 VSWR Hi freq Point: 3801 khz
2:1 VSWR Bandwidth: 341 khz
Fo (Resonant freq) = 3560 khz, 40 ohms resistive, 0 ohms reactance

3500    32,9        1.6
3550    38,0        1.0
3600    44,9        1.2
3650    52,19      1.4
3700    57,30      1.6
3750    63,35      1.8
3800    69,39      2.0

Above table: freq, R,+/- j, VSWR, all taken from the MFJ-269

If the radiation resistance is 26 ohms and I measure 38 ohms feed impedance 
at resonance, then apparently I have 12 ohms of ground loss, for an 
efficiency of:

26/(26+12) or 26/38 = 68%

At least, that's where I'm at for the moment. Time for another 8 radials.

That 1/4 wave transformer (55' of LMR-400 between feedpoint and shack) that 
Tom, W8JI, pointed out, makes a big difference in my confusion...at least it 
all makes sense now.

I think this system is behaving pretty "traditionally" at this point, 
narrower bw is showing lower ground return losses, and the feedpoint Z is 
now believable. Assuming that Devoldere's spec of 26 ohms for a .16 lambda 
vertical inverted L (the rest being more or less horizontal off the top), 
then my ground losses are in the area of 12 ohms with the 8 radials I have 
out so far (66' long)

Thanks again, Tom.

73,

...hasan, N0AN

----- Original Message ----- 
From: "Tom Rauch" <w8ji at contesting.com>
To: "hasan schiers" <schiers at netins.net>
Cc: <towertalk at contesting.com>
Sent: Saturday, September 03, 2005 3:48 PM
Subject: Re: [TowerTalk] Rising Input Z with more radials?


>> Radials  2:1 Fo 2:1 BW Z VSWR @ Fo
>> 0  3340 3522 3920 580 36,0 1.3
>> 2  3354 3524 3774 420 45,0 1.0
>> 4  3419 3533 3741 322 60,1 1.2
>> 8  3445 3550 3742 297 65,4 1.3 (reactance would not go
> lower than 4)
>>
>> The formatting is bad, but in order, left to right:
>>
>> Number of Radials
>> Lower 2:1 vswr point
>> Resonant Freq point (Fo)
>> Upper 2:1 point
>> Bandwidth in kilohertz
>> Impedance (R,+/- j) at resonance as shown on the MFJ 269 .
>>
>> I'm a bit confused by the rising feedpoint impedance with
> increased radial
>> numbers, yet the narrowing down of the 2:1 swr bandwidth
> does indicate
>> decreased losses.
>
> You are measuring through a 1/4 wl transmission line
> transformer, the 55 foot LMR feedline.
> As input impedance at the antenna becomes less, impedance at
> the analyzer is higher because of the "Q" section.
>
> All that aside, I would never depend on bandwidth or
> impedance as an indicator of efficiency unless you have a
> very good feel for how the system behaves.  While there are
> many cases where it is an indicator, there are also many
> cases where the results are counter intuitive and a narrower
> SWR BW and lower impedance antenna is less efficient!
>
> 73 Tom
>
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