> Date: Tue, 19 Jul 2011 19:05:26 -0400 > Subject: Re: Topband: water saturated ground effect > From: [email protected] > To: [email protected] > CC: [email protected]
Guy, 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 <[email protected]> > 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 > > > > > > > > > > _______________________________________________ > > UR RST IS ... ... ..9 QSB QSB - hw? BK > > _______________________________________________ UR RST IS ... ... ..9 QSB QSB - hw? BK
