Interesting observations.
The 2:1 swr points are only being mentioned to show that the bandwidth is
narrowing with increasing number of radials. That makes sense.
What doesn't make sense is the increasing feedpoint Z at resonance in
response to increasing radial numbers. There could be some minor trimming,
but 3 or 4 ohms reactance is probably close enough...the antenna is near
resonance and the resistive component is not likely to change much by
trimming the last 3 or 4 ohms of reactance out. The problem is, I can't
explain the the 65 ohm feedpoint impedance.
Most installations, start with a Z too high (because it includes ground
losses), and as radials are added, the feedpoint Z drops (as the loss
resistance disappears). This is the classical 1/4w ground mounted vertical
case. You know when to stop adding radials when the measured feedpoint
impedance at resonance is equal to the Z of the antenna over a "perfect"
ground.
Another way to accomplish the same thing is to keep adding radials until the
2:1 vswr bandwidth no longer narrows. I think I'm approaching that right
now.
The anomaly I'm trying to deal with is why the feedpoint Z has increased as
I went from 0 to 2, to 4, to 8 radials, when it should have decreased. I
wouldn't care a whit, if the terminal (end point) feedpoint Z was 65 ohms
resistive with no reactive component.
This is a technical element that I want to understand...no further changes
are likely to make any real difference in performance. What is it Johnny
Cochran said, "If the data don't fit, I just won't quit."? vbg
73,
....hasan, N0AN
"Ham op" wrote in message
news
If your antenna is over a PERFECT ground, an EM mirror, then the mutually
coupled impedance between the antenna and it's image antenna gets to be a
bit difficult to calculate or estimate. The Apex and it's image are 80
feet apart. That is 107 degree spacing, and the ends are 77 degrees apart.
The mutual impedance along the length of the antenna changes due to the
difference in phase shift.
Change from perfect reflecting ground to 'real' ground with varying
conductivity and permittivity as a function of length, width and depth and
the problem becomes much more complicated.
Your measurements, in your location, with your installation indicate, to
me, that your 8 radial solution is starting to converge to best solution.
I say this for two reasons. Your 2:1 VSWR bandwidth is narrow and your Z
is approaching 73 ohms.
I would use the eight radial solution and trim the antenna length to your
desired center frequency. Remember, a 2:1 VSWR indicates that 90% of your
energy is going into the antenna.
For the best analysis of your installation, the soil characteristics would
have to be known and controlled for several wavelengths square and
approximately 1/4 wavelength deep. Your radials [counterpoise] are
simplifying and stabilizing your solution.
Roy Llewellen is much better qualified than I to proceed deeper into the
EM Physics regarding all the possible interactions. If the science of
what's happening is your interest then contact Roy off-line [Roy, sorry to
volunteer you without your consent]. If a brief understanding of possible
contributing EM effects is your desire, then I hope this contributes to
your understanding.
Ham Op
hasan schiers wrote:
Question: Where was I taking my measurements.
I started out at the actual feedpoint...then put in about 55' of LMR-400,
and started a new set of measurements in the shack. Here is the latest
data:
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
The formatting is bad, but in order, left to right:
Number of Radials
Lower 2:1 vswr point
Resonant Freq point
Upper 2:1 point
Bandwidth in kilohertz
Impedance as shown on the MFJ 269 at resonance.
I'm a bit confused by the rising feedpoint impedance with increasing
number of radials. That seems backwards to me.
73,
...hasan, N0AN
VSWR shown by MFJ 269 at resonance
"Ham op" wrote in message
...
At what point in the system are you making your measurements?
hasan schiers wrote:
45' high, 25.8 sloping wire at the top, 29 feet high at the far end. #12
THHN Insulated copper wire, stranded.
Predicted Radiation Resistance: 25.8 ohms, very good ground (rich
pastoral, midwest).
With no radials, I get a flat 50 ohm match at 3595 khz. Obviously this
would indicate ground losses of approximately 25 ohms, if I'm thinking
about this right. Also, predicted efficiency would then be 50 %
(25/(25+25), indicating a 3 dB loss. Forgetting about fresnel region
losses, this seems to be better than I had expected. 2:1 vswr bandwidth
is very broad....broader than what I get with EZnec 4.x with a 25 ohm
load in the base.
This means one of two things to me:
Either my ground losses are much higher than the 25 ohms I'm indirectly
calculating, or I have made some sort of conceptual error in thinking
about what the implications are of a 25 or 26 ohm feedpoint. All my
references point toward a 25 ohm radiation resistance for my 42'
vertical x 25.8 ft inverted L (with sloping top wire instead of flat top
wire). What is wrong with my logic here? If the R(rad) is 25 ohms, and I
measure a flat VSWR (on two other meters) at 3600 khz, then isn't the
remaining 25 ohms, ground loss?
I also show about 37 ohms resistance and 0 ohms reactance at 3600 khz
with my MFJ-269, which is really confusing, in that if I have 25 ohms
for Rrad, then I have 13 ohms of ground losses. Further, 37 ohms is
around 1.3 to 1. So I have two other vswr meters showing 1:1 at 3600,
and the MFJ showing 37 ohms. This is a pretty large percentage
difference.
I would be inclined to believe the 25.8 ohms predicted by both EZnec 4
and the Low Band DX'rs Handbook. In any case, I'll put out 4 radials
tomorrow morning and repeat all my measurements, looking for narrowing
bandwidth and lowered input Z as my ground losses decrease.
Ultimately, I'm going to put down 16, 66' radials, in steps of four,
taking measurements of input Z (mfj-269) and vswr bandwidth for 2:1, at
0,4,8 and 16 radials. I'll report what happens as I go along.
Anywho, without any radials at all there are quite a few distant signals
on 80m this evening, that are consistently louder on the newly
installed inverted L, than on my Carolina Windom at 45'. Most signals as
one pans the band, are louder (at 2 hours after sunset) on the C. Windom
than on the radial-less inverted L, but ones from several states away
are equal or better on the radial-less inverted L. Both seem to make
good sense at this point. I sure will be interested to see the effects
of 4 and then 8 and on up radials, but that is going to take a few days,
because I don't want to make radial changes unless I have access to low
angle signals, which only happens at night or just before sunrise.
Tomorrow is radial day. I will be laying out 4 radials 65' long to begin
with. I have resistance and reactance measurements every 50 khz as a
baseline, before installing radials. 4 in the early morning, 4 more just
before sunset, then 4 more the next morning, and the final 4 the next
evening. #14 THHN stranded insulated copper wire for the radials, btw.
Depending on what I end up seeing for "effect" I'll go to 24 or 32
radials by winter....but only if the improvement is both measurable and
"observable on the air"...radials are a pain in the rear (or more
accurately, the knees) to put in.
What fun!
...hasan, N0AN