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Today's Topics:
1. Re: [Bulk] Fwd: WTB Rohn 25G tower insulator (Grant Saviers)
2. Re: [Bulk] Re: Reverse Fed Towers (Drax Felton)
3. Re: Reverse Fed Towers (Dave Thompson)
4. Reverse Fed Towers (n0tt1@juno.com)
5. Re: Reverse Fed Towers (Chuck Dietz)
----------------------------------------------------------------------
Message: 1
Date: Tue, 15 Dec 2015 13:58:17 -0800
From: Grant Saviers <grants2@pacbell.net>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] [Bulk] Fwd: WTB Rohn 25G tower insulator
Message-ID: <56708CF9.8010200@pacbell.net>
Content-Type: text/plain; charset=windows-1252; format=flowed
I have some white UHMWPE that was used as combo thrust and axial
bearings in side arm rotators, sections are over 1" thick. All are
pretty badly crazed and not quite cracked thru after 7 years in the OR
"sun". I didn't make these, so I don't know the grade or supplier of
the material but am cautious about the UV stability of UHMWPE as a
result. I use black material where UV exposure is likely. TIVAR info
on UHMWPE, many grades: http://www.bayplastics.co.uk/polyethylene.htm
Generally, natural UHMWPE is not considered to be UV stable.
I find it hard to machine to tolerance since it is so slippery and
gummy. Very sharp, correct relief tooling is required. Otherwise it
is a very useful material and cheap.
Grant KZ1W
On 12/15/2015 13:31 PM, Hans Hammarquist via TowerTalk wrote:
UHMWPE is available at Walmart (at all places). It's called "TIVAR
GRA0133002037" as sheet Stck, 12 In.x 24 In.x 0.500 In. @ $87.84.
Hans - N2JFS
.
-----Original Message-----
From: Andre VanWyk via TowerTalk <towertalk@contesting.com>
To: towertalk <towertalk@contesting.com>; rcm <robrk@nidhog.net>
Sent: Tue, Dec 15, 2015 3:17 pm
Subject: Re: [TowerTalk] WTB Rohn 25G tower insulator
I will use these if all fails. UHMWPE material will outlast
fiberglass in
the sun too.
73's
NJ0F
-----Original Message-----
From: rcm
Sent: Tuesday, December 15, 2015 1:40 PM
To: towertalk@contesting.com
Subject: Re: [TowerTalk] WTB Rohn 25G tower insulator
http://www.wb0w.com/tower_insulators
On 12/15/2015 01:06 PM, Andre VanWyk via TowerTalk wrote:
I am getting parts together for a 160m vertical, using Rohn 25G
sections.
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------------------------------
Message: 2
Date: Wed, 16 Dec 2015 02:02:07 +0000 (UTC)
From: Drax Felton <draxfelton@gmail.com>
To: Peter Voelpel <dj7ww@t-online.de>, Grant Saviers
<grants2@pacbell.net>, <towertalk@contesting.com>,
<wosborne44@gmail.com>
Subject: Re: [TowerTalk] [Bulk] Re: Reverse Fed Towers
Message-ID:
<AA176A7399F20B16.91FC6671-6B2D-4ACA-9EBB-B6B6989B25D6@mail.outlook.com>
Content-Type: text/plain; charset=UTF-8
It had no effect on that stuff when I did it. ??
Sent from Outlook Mobile
On Tue, Dec 15, 2015 at 1:26 PM -0800, "Grant Saviers"
<grants2@pacbell.net> wrote:
Curious as to how one manages all the rf induced into yagis, rotators,
coax, control cables, etc when an antenna loaded tower is QRO excited.
Also, how the same cables are decoupled from the tower at whatever
levels are appropriate and what is done at the shack end if anything.
Grant KZ1W
On 12/15/2015 12:55 PM, Peter Voelpel wrote:
I use a system like that.
My tower is loaded with some yagis and 46m high at the top.
I feed it at the 25m level against a single sloping radial.
The inner conductor is connected to the radial, the shield to the
tower.
The connection point was simulated with EZNEC before and was right at
less
then 1m difference.
The antenna works perfectly well for me, SWR2 band width is about
200kHz and
fine tuning can be done at the radial.
It is also very good on receive, picks up much less noise then the
previously used ground fed T-vertical.
>From distances beyond 1000km it is already better then the 30m high
inverted-vee.
For cq160 I usually add an inverted-L reflector for directivity to
VE/W.
If you are interest I will email the EZNEC file.
73
Peter
-----Original Message-----
From: TowerTalk [mailto:towertalk-bounces@contesting.com] On Behalf
Of
wosborne44@gmail.com
Sent: Dienstag, 15. Dezember 2015 18:46
To: towertalk@contesting.com
Subject: [TowerTalk] Reverse Fed Towers
I have a tower that has a base that is in concrete and grounded. I
would like to make it a vertical without installing insulators. Has
anyone used elevated radials with reverse feeding, i.e., connecting
the
center conductor to the radials and the shield to the grounded tower?
I
see this in the ARRL handbook but I cannot seem to make a model of it
work. Any help would be welcome.
Thanks,
William Osborne--K5ZQ
270-205-9565
Wosborne44@gmail.com
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------------------------------
Message: 3
Date: Wed, 16 Dec 2015 01:30:06 -0500 (GMT-05:00)
From: Dave Thompson <thompson@mindspring.com>
To: towertalk@contesting.com
Subject: Re: [TowerTalk] Reverse Fed Towers
Message-ID:
<7743743.1450247406879.JavaMail.root@mswamui-valley.atl.sa.earthlink.net>
Content-Type: text/plain; charset=UTF-8
Tom N4KG explained reverse feeding a tower in as I recall the May 1994
QST.
Tom does a good job of making it easier and yes, this is an elevated
radial system. My main concern was not getting the radials far enough
off the ground
to avoid ground loss. ON4UN told me the radials needed to be 40 feet
off the ground to be safe. There was a report by Christman released at
Dayton in the early 1980's where he built a half wave vertical dipole
that required isolators to be effective. K4PI looked at the design and
told me it was too complicated.
I built one using Spi-Ro short radials and using the MFJ 259 found that
my 71 foot tower top loaded was matched at 1830 at 18 feet. A couple
of others used the design to make effective 160 or 80 meter verticals.
Dave K4JRB
------------------------------
Message: 4
Date: Wed, 16 Dec 2015 07:23:40 +0000
From: <n0tt1@juno.com>
To: TowerTalk@contesting.com
Subject: [TowerTalk] Reverse Fed Towers
Message-ID: <AABMHCFFAAKT2F2A@smtpout04.vgs.untd.com>
Content-Type: text/plain; charset=us-ascii
On Tue, 15 Dec 2015 17:45:39 +0000 wosborne44@gmail.com writes:
I have a tower that has a base that is in concrete and grounded. I
would like to make it a vertical without installing insulators.
Has
anyone used elevated radials with reverse feeding, i.e., connecting
the
center conductor to the radials and the shield to the grounded
tower?
Hi William,
Yes, I did that this Summer/Fall, for a 7mhz antenna.
Details, details.....
I have a 70-ft BX tower and I installed
12 radials (taking Rudy, N6LF's advice) that are
insulated from the tower in groups of
4 aluminum wires.
BTW, it's interesting to note that our favorite
frequency standard, WWV, has 9 radials
on their vertical dipole antennas.
The radials attach to a "buss" that
is insulated from the tower legs with a short,
ceramic "strain" insulator. The insulator
is clamped to the tower via a small bracket
and two SS hose clamps. Each "buss" is
connected to a center insulator via 3 wires
stripped from RG8 type coax and wrapped
in tape to prolong their life. Each radial terminates
in SS hardware on the "buss" to provide a mechanical
connection. Each radial electrically connects to the "buss"
via a homebrew wire clamp and a short copper jumper
with AL/CU terminals.
Connection to the tower is at 3 places via a 3/16" thick
aluminum plate, triangular in shape, with 3 "arms" that are
made adjustable in their length. At the ends of the arms
I have a block of aluminum with slots cut into them that
clamps onto the heavy sheet metal legs of the tower with
a couple of SS screws. The top of the plate holds the
center "beehive" type of insulator and the bottom
holds a PVC electrical box. Inside the box I have a
1:1 balun/choke wound on a FT240-61 (gasp!) toroid.
The output of the choke connects to the beehive insulator
and the supporting aluminum plate. The "leads" are reversed
so that the radials are at DC ground via the shield of the
coax matching section that feeds the input of the balun.
The shield is grounded at the base of the tower. The
matching section is the usual 1/4-wave length of two 75-ohm
RG-11 coax in parallel.
Up on the tower, with a antenna analyzer connected to
the input of the balun, I found, as expected,
no noticable change in input impedance when I shorted the
coax shield to a tower leg. (Remember, the shield connects
to the radials.) So, there's no issue with running the coax
feed over to a tower leg (inside the tower) and then down to
the base.
About radial and tower lengths....I did start to model the
antenna using EZNEC 6.0+. It was, for me, a waste of time
because of all the angles of the tower, the 3 jumpers that
feed the radial from the center of the tower, etc etc. A complicated
mess! So, I started with a vertical tower length of 234/f and a little
longer
for the radials...something like 34 feet each. I expected
that the vertical radiator length would be shortened
by about 6-10%. It turned out that it was much more than that.
I ended up about 7 ft shorter than the 234/f length. I ended
up with the radials being about 33ft each.
I measured the current in each wire on the
"output" of the choke/balun and found that they were
virtually equal at resonance. Per Rudy's white paper,
(Google it) it's important to balance the currents in this antenna.
The instrument I used was a homebrew current meter.
I powered up the antenna with my MFJ-269 antenna analyzer placed
right at the input of the choke/balun and the current meter
probe was clamped onto each output lead of the choke/balun....
obviously not at the same time.
I attached some photos for you.
Well, that's about it except to say, that I'm pretty sure that
the radials could be attached directly to the tower legs and then
feed the vertical section fed with a gamma match. That's not
been tried here, but it's on my list of things to do for another
vertical antenna.
73,
Charlie, N0TT
------------------------------
Message: 5
Date: Wed, 16 Dec 2015 06:16:27 -0600
From: Chuck Dietz <w5prchuck@gmail.com>
To: Dave Thompson <thompson@mindspring.com>
Cc: "towertalk@contesting.com" <towertalk@contesting.com>
Subject: Re: [TowerTalk] Reverse Fed Towers
Message-ID:
<CAOk0j1-33W7hpvFxr+Vg5R56Sr7MKRA8m=r8DMKwUyz4N3aopQ@mail.gmail.com>
Content-Type: text/plain; charset=UTF-8
To avoid RF in the shack, bury the coax/hardline.
Chuck W5PR
On Wed, Dec 16, 2015 at 12:30 AM, Dave Thompson
<thompson@mindspring.com>
wrote:
Tom N4KG explained reverse feeding a tower in as I recall the May
1994
QST.
Tom does a good job of making it easier and yes, this is an elevated
radial system. My main concern was not getting the radials far
enough off
the ground
to avoid ground loss. ON4UN told me the radials needed to be 40 feet
off
the ground to be safe. There was a report by Christman released at
Dayton
in the early 1980's where he built a half wave vertical dipole that
required isolators to be effective. K4PI looked at the design and
told me
it was too complicated.
I built one using Spi-Ro short radials and using the MFJ 259 found
that my
71 foot tower top loaded was matched at 1830 at 18 feet. A couple of
others used the design to make effective 160 or 80 meter verticals.
Dave K4JRB
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------------------------------
Subject: Digest Footer
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------------------------------
End of TowerTalk Digest, Vol 156, Issue 54
******************************************