Hi Tony,
There's a VERY serious flaw in your plan. That flooded Commscope RG6,
which I use extensively for RX antennas, does NOT do well with sharp
bends. The center conductor, which copper-clad steel, shorts to the shield.
There's another flaw. I suspect you're looking at SWR to define
bandwidth. That's a poor indicator for a directional antenna, especially
one that depends on coax lines to provide the required phase
relationships. That's because the phase varies with frequency AND the
length of the line, so when you triple the length of the line, you
narrow the bandwidth over which the antenna exhibits it's desired
directionality.
73, Jim
On 1/29/2019 8:18 AM, N2TK, Tony wrote:
Jim,
I would like to continue on with this 4-sq subject but for 80M. For many years I
have been using an elevated feed (10' high) with elevated radials. I have been using
Commscope flooded RG6 wit 100 beads on the end of the coax at the feedpoint. I want
to bury the feedlines. I made up 3/4 wave feedlines with big clamp ferrite cores
with several #31 cores. I tried these feedlines. The only thing I noticed was a very
slight narrowing of the bandwidth. In the spring when the ground thaws here in
up-state NY I will bury the lines. Based on your latest updates it seems I should
change the big clamp to 4" cores. I would like to use the RG6 as it would be a
continuous run. I am sensitive to what you said about sharp bends. The plan would be
to make up some kind of fixture like in your Fig 8 so I can ensure the turns don't
overlap and the bends are not too sharp. Can you think of any negative aspects of
doing it this way over making up some RG400 chokes?
Tnx for any feedback
N2TK, Tony
-----Original Message-----
From: RFI <rfi-bounces@contesting.com> On Behalf Of Jim Brown
Sent: Tuesday, January 29, 2019 1:55 AM
To: john@johnjeanantiqueradio.com; Rfi List <rfi@contesting.com>
Subject: Re: [RFI] Common mode choke on 4 Square feedlines without skewing
phasing?
Hi John,
As Gedas has observed, some of these specialty coax cables very from one mfr to
another. I must plead a certain degree of ignorance -- until needing something
with the specs of RG400 for these chokes, I'd never used any of these premium
teflon cables.
Thinking about power requirements, each vertical in the array gets only
1/4 of the TX power, which, if there were no feedline loss is 375W in a legal
limit system. Anyone who does more than that gets no help from me.
:) Depending on length of the home run to the shack, my guess is that you're
hitting the antenna with no more than 350W, and probably closer to 325W.
I'll leave the choice of coax up to the user, but it seems to me that virtually
any practical coax should handle that much power with a lot of headroom.
Another piece of information. I just pulled out the data for the VNA
measurements I did several years ago for the RG11 that I bought from Davis RF.
VF converges to 0.8423 at VHF, and VF at 160M is 0.831. Loss on 160M is
0.167dB/100 ft. DX Engineering says their cable is 0.78.
Assuming the controller in the geometric center of the array, it's 94.5ft to
each vertical. The lines to the verticals are specified as a quarter wave,
which is 105' at 1830 with VF=0.78, and 111' with the Davis RF RG11. This
means that using Davis RF RG11 gives you 6 ft more coax to play with while
still maintaining proper phasing, so it seems to me that there should be more
than enough to rig to the antennas and the controller and still allow you to
subtract enough to account for adding the chokes.
At that power level, the 10K ohm Rs you get from 18 turns on a 2.4-in o.d. #31
seems quite conservative for dissipation unless something breaks pretty badly.
My spreadsheet says 4.8 ft to wind the choke, allowing a total of 8-in for
leads.
Hope this helps.
73, Jim K9YC
On 1/25/2019 5:51 PM, John K9UWA wrote:
Hi Jim
Yesterday you mentioned a 75 ohm small diameter coax that had a small
bend radius to wind 80m and 160m 4 square chokes with. when I looked
that coax number up it came up as a 50 ohm cable so after looking
about I found some 75 ohm small diametercoax'es but none that had
small radius bending or ones that would handle 1KW PLUS for 160m and
80m. My issue being that I have a K8UR style 4 square for 160m which
means I am pulling this vertically mounted InvertedV antenna out about
95 feet from the supporting tower where both weight and mass being
that the station is located in Northern Indiana where we have Wind Ice
and Snow loads to think about. So yes I would love to put 4 nice
chokes at the feedpoint of these 4 antennas. We just need a coax
product number that will hold up in the above situation as well as
fitting onto hopefully a single FT240 mix 31 core. The RG302/U fits
all of this except the possible migration of the center conductor due
to bending radius?
thanks
73
John k9uwa
On 1/24/2019 5:38 AM, john@kk9a.com wrote:
I use RG302/U for 75 ohm chokes.
Hi John,
I would be concerned about bend radius for tightly wound turns on
the 2.4-in o.d. toroids with the solid steel center of this cable.
For other applications it looks fine.
73, Jim K9YC
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