Thanks, Carl! Well, if you' heard a RX peak at 1770 KHz, it seems that you
are awfully close! Just needs a little careful "tweaking" I would think. (I
ass'ume that you are probably shootinf for something around 1830 KHz. BTW -
the higher you tap on the tower, the smaller the series capacitor needs to
be, since the increasing series inductive reactance will require increasing
capacitive reactance (lower C) to cancel it, so it sounds like you can use
the 160 pF capacitor for your series tuning C.
I haven't used my MFJ 259 in a while, so I would need to get it out and
review its operation but when you tuned down to 68 ohms impedance whtn
tapped at 90', I expect that's where the impedance became pure real at 68
ohms. As an additional check you can drive the gamma wire with a little
power from your TX or the MFJ and tune the series capacitor for minimum SWR.
Sounds like it should come in around 1.4. If it does, you're done. Just bolt
everything down and enjoy. Clearly, if the 46' tap pointis showing24 ohms
real that's way too low on the tower for your tap point! It sound
like90]isprobably the point you want!
GL, Carl!
Have fun!
73,
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Carl
Braun
Sent: Saturday, February 15, 2014 3:11 PM
To: Charlie Cunningham
Cc: topband@contesting.com
Subject: Re: Topband: Still in search of resonance - Detailed Observations
and Calculations
Charlie
Thank you for your work and insight.
It appears I've I may have provided some incorrect info.
When I tapped the tower at 90' I used the 160pf variable cap to get down to
the 68 ohm impedance measurement and, yes, it heard well with what appeared
to be a peak at approx 1770kc I never transmitted there. I've only
transmitted with the system when I had a tap at 46' where I saw 24 ohms and
X=0 with the variable. Ap on series. Then I installed a 22 to 50 ohm Unun
and made the contacts to east coast stations.
I believe I have plenty of capacitance on hand if I tap the tower at 90' but
given the 68 ohm reading at 90' with the variable cap and the 24 ohm reading
at 46' with the variable cap don't you think the best bet would be my 67'
tap point? Even if it's still a bit low in resistance at that point i could
add a bit of parallel C in conjunction with the series C to bring the
antenna to 50 ohms+j0?
Please advise and thank you for the most enjoyable technical conversation.
Carl AG6X
Sent from my iPhone
On Feb 15, 2014, at 9:14 AM, "Charlie Cunningham"
<charlie-cunningham@nc.rr.com<mailto:charlie-cunningham@nc.rr.com>> wrote:
Well, here it is with the re-built loss table, Carl
73,
Charlie, K4OTV
From: Charlie Cunningham [mailto:charlie-cunningham@nc.rr.com]
Sent: Saturday, February 15, 2014 11:35 AM
To: 'Carl Braun'; '160'
Subject: RE: Topband: Still in search of resonance - Detailed Observations
and Calculations
Hi, Carl
I did a bit of further investigation and work on your problem.
I think you are done, as follows:
1.0 Just tap the Skyneedle at 90' and tune out the series inductive
reactance with a variable capacitor, leaving you with 68 ohms real at the
bottom of your drop wire. Great match! VSWR on 50 ohm feed cable of
1.4:1.
2.0 Now, let's assume you have 250 feet of Belden 8237 (RG-8)
feeding the bottom of the drop wire. Losses are as follows:
Line/Load
Line type: Belden 8237 RG8
Line length 250?
Frequency 1.8 MHz
Load SWR 1.4:1
Power In 100W
Results:
Matched Loss: 0.577 dB
SWR Loss 0.029 dB
Total Loss 0.606 dB
Power Out 86.982 W
Note that the excess loss due to the SWR on the cable is 0.029 dB, out of a
total loss in 250? of RG-8 of 0.606 dB
Note the ?flat-loss? or ?matched loss? of the cable (at 1:1 VSWR) is 0.577
dB. So there?s no real point in struggling to get to exactly 50 ohms real
at the bottom of your drop-wire to recover 0.029 dB of loss in 250? of
cable! Your 68 ohms is just fine! Just match tle line at the transmitter
end and accept the modest 1.4:1 VSWR at the load end.
As you observed, when tapped at 90? the tower ?heard? very well and you made
some contacts with your FT-1000D barefoot.
So, it surely appears that you have a very good, well-matched antenna when
you ?tap? at 90? and tune out the series inductance of the gamma match in
the normal way using a series capacitor. Just tune for X=0 at th bottom of
the drop wire, connect the feedline and match the feedline at the
transmitter and enjoy!!
Of course, with a tower that tall, you probably want a ?spark gap? and/or a
gas-tube at the feed-point and sopme sortof static bleed to defend agains
static charge and lightning!
GL!
Have fun!
73.
Charlie, K4OTV
-----Original Message-----
From: Topband [mailto:topband-bounces@contesting.com] On Behalf Of Carl
Braun
Sent: Friday, February 14, 2014 6:15 PM
To: '160'
Subject: Topband: Still in search of resonance
List
Some of you may have followed my efforts in trying to shunt feed my 90'
Tri-Ex Skyneedle with 20 meter yagi at 93'. I'm still unable to find any
sort of resonance point on the tower. To refresh everyone's memory here are
the specifics:
90' Skyneedle that is 12" round at the base and 4" round at the top
13' of mast out the top
5 element Telrex 20M monobander mounted at the 93' level. No other antennas
on the tower
1 ½" copper pipe as a radial ring that surrounds the concrete base that
measures 4' x 8' rectangle. Three 8' ground rods are connected to the
radial ring via 1" copper strap that is .125 thick.
Currently I have 27 14AWG insulated wire radials. Most of the radials are
20' to 50' long with three at 90 to 120' long and four of them connected to
my 40M vertical array which have 100 count radials 50' to 100' each.
The tower is grounded to each ground rod via 1" copper strap .125" thick
and, as mentioned above, the ground rods are connected to the radial ring
with the same strap with copper clad stainless screws.
When I bolted the gamma arm to the tower at the 90' height I dropped a
single 14AWG wire to the ground where my FLUKE meter read ZERO ohms between
the radial ring and the end of the gamma wire with no fluctuations so I'm
confident that I have good continuity throughout the tower.
Here are the readings that I saw on the MFJ analyzer with the gamma arm
mounted at the (4) points on the tower that are available...
With the gamma arm mounted at 90' and 36" spacing I saw 425 ohms at the end
of the drop wire on the MFJ
With the gamma arm mounted at 67' and 36" spacing I saw 380 ohms at the end
of the drop wire on the MFJ
With the gamma arm mounted at 46' and 36" spacing I saw 240 ohms at the end
of the drop wire on the MFJ
With the gamma arm mounted at 28' and 36" spacing I saw 120 ohms at the end
of the drop wire on the MFJ
At all of these points I was able to knock down the R with my honkin' 1050pf
cap to some resonance sort of resonance at 1.825 MHz but, as most everyone
has indicated, I should be able to find a 50 ohm tap somewhere on the tower.
I can't find it.
When I had the gamma arm mounted at the 90' level. I was able to put my baby
variable 160pf inline to bring the 425 ohm impedance down to about 60 ohms
and the antenna heard very well; especially on the 1700 KHz broadcast band,
with a 2.4:1 Vswr. Similar results could be seen at the other levels too as
long as I brought the R down with a variable cap. Yesterday, with the gamma
arm at the 46' level (and 240 ohms on the MFJ) I was able to put the big
variable inline to bring the reading to 24 ohms with a TRUE X=0. With a 22
ohm to 50 ohm UNUN, I saw 1.3:1 Vswr on the output of the UNUN. I worked a
W2 in NJ and a W4 in Florida with just the 1000D. BUT...again...I'm
bringing the R down with the capacitor...not finding 50 ohms anywhere on the
tower.
Is my radial field so poor that I'm seeing these goofy readings?
Is the single 14AWG too thin causing goofy readings?
I'm back to scratching my head.
Comments from the list?
Carl AG6X
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
_________________
Topband Reflector Archives - http://www.contesting.com/_topband
|