Re: 80m Inverted L Initial measurements

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" <hamop@xxxxxxxxxxx> wrote in message
news:xMudncOoYMZYDYTeRVn-jw@xxxxxxxxxxxxxx
> 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
>


.

Relevant Pages

  • Re: 80m Inverted L Initial measurements
    ... The vswr meters and the mfj all read the same ... with increasing numbers of radials.) ... >>indicate ground losses of approximately 25 ohms, ... >>morning and repeat all my measurements, ...
    (rec.radio.amateur.antenna)
  • Re: 80m Inverted L Initial measurements
    ... >THHN Insulated copper wire, stranded. ... 25.8 ohms, very good ground (rich pastoral, ... >With no radials, I get a flat 50 ohm match at 3595 khz. ... I have resistance and reactance measurements every 50 khz as a ...
    (rec.radio.amateur.antenna)
  • Re: 80m Inverted L Initial measurements
    ... 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. ... a 2:1 VSWR indicates that 90% of your energy is going into the antenna. ... Your radials are simplifying and stabilizing your solution. ... 25.8 ohms, very good ground. ...
    (rec.radio.amateur.antenna)
  • Re: 80m Inverted L Initial measurements
    ... THHN Insulated copper wire, stranded. ... Obviously this would indicate ground losses of approximately 25 ohms, if I'm thinking about this right. ... 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). ... 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. ...
    (rec.radio.amateur.antenna)
  • Re: using coax shield to create a loading coil ?
    ... I just wanted to systematically build up the antenna, ... of radials -- something that is pretty well known. ... How is the lumped inductance set of 4 Ohms and 1750 Z? ... loading coil raises the input impedance to 11.5 ohms, and gain +2.6 ...
    (rec.radio.amateur.antenna)
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