Posted by
Jim Brown-10 on
Dec 09, 2018; 8:14am
URL: http://elecraft.85.s1.nabble.com/Flumoxed-tp7646765p7646971.html
Several reasons why this is a bad idea. First, resonant dipoles are a
much closer match to 50 or 75 ohm coax than to 300 ohm line. Low dipoles
are closer to 50 ohms, higher ones closer to 75 ohms.
Second, the MOST important place for a choke is at the feedpoint. The
feedline is part of the antenna until it hits the first choke! If, for
example, the feedline is 60 ft long and the only choke is at the shack,
that 60 ft of feedline is picking up noise and can be de-tuning your
antenna. Additional chokes along the line can prevent the feedline from
interacting with nearby vertical antennas, and can further reduce
feedline noise.
Third, N6BV wrote an excellent piece for QST several years ago observing
that it's VERY easy to fry a common mode choke on a line with high
SWR. This happens when the choke is at a high current point on the
transmission line, overheating the line and the ferrite core.
I will soon publish a new Choke Cookbook for chokes wound on a single
#31 2.4-in o.d. core with RG400, a #12 teflon pair, and a #12 THHN pair.
I am no longer recommending chokes wound with RG8/11/213 size coax, for
reasons that will be detailed in the applications note that comes with
the "cookbook." I've been beta testing THHN and enameled pair chokes for
years; recent work shows that that RG400 and teflon #12 are the better
choice.
An 80/40 fan dipole will perform as well as separate 80 and 40 dipoles
at the same height -- the only compromise is that SWR bandwidth is
reduced by about half on 40M, but a fan dipole cut for the center of the
band is still below 2:1 at the band edges.
As to sag -- height IS a good thing. Modeling shows that horizontal
antennas, it takes a change of 5 ft on 40M for between 30 and 70 ft, and
10 ft on 80M for antennas twice that height to make a difference of 0.9
dB. That work is presented in this peer-reviewed paper.
http://k9yc.com/AntennaPlanning.pdf The discussion of rigging height
begins around page 10.
Yesterday in this thread someone questioned my statement that the
difference in field strength between a horizontal dipole and an inverted
Vee with it's apex at the same mounting height was in the range of 1-2
dB. For another project, I just modeled an inverted Vee for 80M with
it's apex at 46 ft and ends at 17 ft. The difference was about 1.5 dB as
compared to a horizontal antenna at 46 ft. Bringing the ends further
down will, of course, increase that difference. I didn't model it, but
3dB seems a reasonable number. But remember, with antennas, higher is
usually better. :) When we use Inv Vees for portable setups, we always
try to rig the ropes for the ends of the antenna as far away and as high
as possible.
73, Jim K9YC
On 12/8/2018 8:00 PM, Bob McGraw K4TAX wrote:
> The 80 / 40 M fan dipole fed with 300 ohm transmitting line {The
> Wireman #562} and then use the balun at the point where the feed line
> enters the house would be preferable.
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