Stan is correct. Site specific factors can dramatically affect antenna
performance. Using computer simulations/models is a start, but one also needs
to consider that free space approximations will be altered due to height above
ground, proximity of buildings, downspouts, trees, local terrain, and a host of
other factors. Some antennas are more susceptible to these influences than
others. Hence, I consider the theoretical approximations along with the
experiences of others, hopefully based upon instances where good, bad, or
average band conditions contributed to the observations.
Let's all enjoy the hobby!
73,
Evan, K9SQG
-----Original Message-----
From: Stan Nafziger <kf4...@gmail.com>
To: K9sqg <k9...@aol.com>
Cc: damienmannix <damienman...@hotmail.com>; Drakelist <drakelist@zerobeat.net>
Sent: Wed, Sep 11, 2013 11:46 am
Subject: Re: [Drakelist] MN-2700/B-1000 use
Hello Damien,
I also have an MN-2700 with the B1000 balun so I can share my experience. I
have two ladder line (450) fed inverted-v's with the centers at 30 ft and
oriented 90 degrees to each other. One is slightly lower than the other to
avoid physical contact of the wire and the centers are spaced about 4 feet
apart. One is cut for 60M and the other for 75M. The two feed lines drop
vertically for about 20' parallel to each other spaced about 4' apart. I have
a patch panel in the shack to select the antenna needed. Because the vertical
feed line will radiate to some degree, there will be some RF induced in the
adjacent feed line so it should remain unterminated when not in use. The
MN-2700 will tune each antenna flat from 3.5 - 29 Mhz. I generally run about
500 watts on 75m and have no issues.
I have no idea how this antenna system would look on paper and don't really
care...it works! Like Evan said, experiment and have fun. "Paper" and
antennas and "real" antennas rarely perform the same.....too many variables to
consider.
Hope that helps.
73,
Stan, KF4BY
On Wed, Sep 11, 2013 at 11:12 AM, K9sqg <k9...@aol.com> wrote:
Damien,
As a start, I would go to various books and online articles regarding the
impedance of your antenna (resistive AND reactive components) considering its
length, height above ground, AND the bands that you plan to operate. That will
suggest whether 450 ohm or 300 ohm twin lead is more appropriate.
Off hand, I would not expect a large amount of degradation due to the twin feed
passing through pine trees. I have some antennas fed with 300 ohm window line,
although I have used 450 ohm previously, and I have an 8 inch standoff where it
passes close to the trunk of a tree BUT this is to prevent rubbing of the
feedline against the tree. In addition, I have committed several sins with
regard to routing of the twinlead. It goes through two 90 deg bends as it
enters the house through some weatherproof quick disconnects (for when storms
approach), another 90 deg bend when it first enters the house, follows water
pipes and heating ducts spaced about 6-8 inches from the twinlead, three more
sharp 90 deg bends as it goes from the basement, up through the wall, and into
my shack, with yet another 90 deg bend as it enters another quick disconnect.
This last couple of bends put the twinlead an inch or two away from a heating
duct for about four inches. While I have two such feedlines they are spaced
about a foot apart as they run through the house. Both feedlines have
approximately one twist per each three feet of length. Inside, I use electric
fence insulators to space the twinlead away from the floor joists. In spite of
all these violations of preferred practices, the antennas work well to my
satisfaction. It is not uncommon to hear unsolicited reports of "You're the
loudest signal on the band" or "You are consistently putting in a good signal
to my QTH." The installation is indeed a compromise but it is the best I have
been able to do.
Through experimentation, I see no interaction by having the feedlines about a
foot apart for a length of approximately 50 feet inside the house. The two
antennas are concentric loops, one for 40 meters at a height of about 12 ft and
the other is a loop for 80 meters at a height of about 20 ft. Shorting the
unused antenna, grounding it, etc. does not seem to affect the SWR on the
active antenna in use; whether it affects the radiation pattern is another
issue but it is of little concern since I'm pleased with the performance of the
antennas and feedlines.
As a side note, there are two reasons why I use 300 ohm transmitting style
window line. (1) Analysis of my antennas indicated that the 300 ohm was a
better match than 450 ohm although I could see no change in performance between
the two. (2) The 300 ohm window line has a lower visual cross section and it
pleases the xyl; happy wife, happy life. There is likely to be some pickup
between the two feedlines but it seems to be minimal.
Experimentation with antennas can be as much fun as it is educational. A
compromise antenna in the air works a lot better than the perfect antenna
design on paper. Enjoy the experimentation, and enjoy the hobby. Please share
your experiences with others.
73,
Evan, K9SQG
-----Original Message-----
From: Damien Mannix <damienman...@hotmail.com>
To: drakelist <drakelist@zerobeat.net>
Sent: Wed, Sep 11, 2013 10:46 am
Subject: [Drakelist] MN-2700/B-1000 use
I have just been fortunate enough to obtain an MN-2700 with Balun.
Sorry if this is slightly off topic, and I hope it is not a silly question, but
if I feed an antenna 150 foot away with 450 ohm twin feed, will it matter if it
passes through two mature pine trees whilst on its way? Also, if I want to
feed two antennas can I run two 450 ohm twin feeds beside each other or will
they interact?
I have looked at endless antenna books but cannot find the answers.
Regards
Damien G3XER
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