Topband: water saturated ground effect MA160V
BRYON PAUL n0ah VEAL
bryonveal at msn.com
Tue Jul 19 22:08:09 PDT 2011
> Date: Tue, 19 Jul 2011 19:05:26 -0400
> Subject: Re: Topband: water saturated ground effect
> From: olinger at bellsouth.net
> To: bryonveal at msn.com
> CC: topband at contesting.com
FB response to pick up on- We'll be in Costa Rica with the Youth DX Association from July 21-25th, so please look for us all- and I'll enjoy reading any follow up on this as time goes on-
Today, all test were at the feedpoint, without the UnUnm including trying two different short feedlines of a few feet- They both showed the same- the minimum SWR is finally creeping up a bit, from 1:1 yesterday to 2.1:1 today- then it rained again- I feel the ground soaked earth is causing some interesting measurements on the antenna. Very unexpected. In a few days, once we return to a scortched earth, it will be interesting to see what happens. If the SWR goes back up to 5:1 or so without the UnUn in line as things dry out, that is, just measured at the feed point, this would be an interesting topic to discuss- If it remains low, then I'll just look at the loading coil - but since it rose up some today, I really feel the drying out saturated ground is messing with things- thx for all the awesome responses- This is a good topband antenna for small back yards from our view-
> The change in SWR with the dampness is an indication of the
> insufficiency of your radial system compared to a dense, commercial
> grade radial system.
I disagree here in that adding the radial system incrased the SWR from 1:1 to 5:1 when first installed last year showing it did make a significant improvement in the antenna's efficiency-
> 26 buried radials would not be sufficient in any event, to insure
> complete independence of the antenna from the wide variation in
> ground-related effects. For there are either too few of them if they
> are quarter wave, or too short if 26 radials are properly dense close
> to the antenna. Even if all your radials are 1/8 wave which borders on
> dense, use of only 1/8 wave in dirt as variable as you describe would
> not prevent seasonal variation, as you would still have significant
> ground interaction beyond the 1/8 wave. Further, if the radials are
> not equal and evenly spaced, some portion of the ground field
> cancellation benefit will be lost, the exposure to ground losses
> increased, and therefore the variation is increased.
Verdict is out here for us-but i appreciate the explanation-
> Further yet, given the shortened (30 to 36 feet) antenna, the
> performance basically is 95% determined by the ground treatment, which
> is why you can have huge variation with ground changes. Worse yet,
> empirical evidence continues to accumulate that in typical ham
> situations, and with reference to sky wave, that even the mighty NEC4
> modeling methods ***UNDERESTIMATE*** ground losses UNTIL a DENSE,
> commercial grade radial field is in use. Model results seem to play
> truer to performance if one always starts with POOR ground assumptions
> until a more optimistic estimate can somehow be PROVEN. Guess or hope
> doesn't cut it.
Rersults do- we went from an average of 50 QSO's to between 500-600 in last year's ARRL and CQWW 160M contests using 100 watts- Record distance is making the ZL8X log with 100 watts. No sky wave asperations here om- we DX (-:
> The minus 6 dBi on the DX engineering page could easily be minus 9 or
> even minus 12.
No disagreement here->
> There is a simple rule that is emerging with respect to on-ground or
> buried radials: If you can't do dense and quarter wave, don't do
> on-ground or buried. I realize that creating such can be beyond
> reach, but less than the full monty on ground based radials is far
> lossier than most realize. If that's all you can do, you're stuck
> with it, and you will be victimized by the dirt, high losses when the
> SWR is good and broad, and lower losses when the SWR is poor (and
> matching is difficult).
Ouch- then why does the SWR go from 1:1 to 5:1 with the added radials exposing the impedance mismatch? I have used above and buried radials- it is true that until this antenna, the above ground radials always showed more antenna efficieny improvement issues than buried- with dogs and a small yard now, we bury everything-
> Solving your SWR will not solve your performance. Solve the ground
> and then match that.
I disagree here- Seeing the higher SWR with the added radials, shows the antenna becoming more efficient- using the UnUn to lower the minimum SWR at resonance gives the antenna a broader bandwidth- my 2 cents (-:
> 73, Guy.
> On Mon, Jul 18, 2011 at 8:31 PM, BRYON PAUL n0ah VEAL <bryonveal at msn.com> wrote:
> > On my Cushcraft MA160V, the resonant SWR point has lowered from almost 5:1 to 1:1 without the Amidon unUn in line. All 26 of the buried radials are attached. The main difference is that the normally dry ground is saturated from 6 inches of rain in the past two weeks. Can the water saturated soil really make this much of a difference? The last time this antenna showed this good of an SWR, was when I had no radials attached, in dry dirt, as expected- Of course attaching radials only began to show the true impedance mismatch, thus the need for the Amidon UnUn to match the coax to the antenna- Bottom line, the antenna is acting like I disconnected the radials, which are very much intact- My guess is that as the soil drys out, my SWR at sresonance will once again rise, and i will need the UnUn again for matching reasons- does this sound right>?
> > 73 Paul N0AH
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> > UR RST IS ... ... ..9 QSB QSB - hw? BK
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