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[TowerTalk] Do Log-Periodics Stress the Selectivity of the Receiver?

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Subject: [TowerTalk] Do Log-Periodics Stress the Selectivity of the Receiver?
From: DavidC" < (DavidC)
Date: Sat, 27 Nov 1999 10:17:39 -0500

   Thanks for your comments!  I'm afraid I didn't clarify all of the
parameters of my question to the log periodic manufacturer.

   My comparison was only to the Force 12 trapless design, not beams
whose traps represent frequency-specific tuned circuits that somewhat
screen-out non-design frequencies.

   You do make a good point though.  Trapped beams would have the
advantage of tuned circuits at the point of signal gathering in the air!

   Has anyone done a A-B comparison of a log to a non-trapped beam
where there is not the advantage of tuned traps helping to screen out non-
design frequencies?

- Thanks! & 73, DavidC  K1YP

> On Fri, 26 Nov 1999, DavidC wrote:
> > Does not the log-periodic challenge the receiver with a larger number
> > of high-level signals?  Does this not present a greater likelihood of
> > selectivity and filtering problems, especially in receivers with broad
> > front ends?
> and from a log periodic manufacturer:
> > I don't think this would be the case at all.  In fact it could be just
> the
> > opposite.  Your theory is reasonable but.......  Assume a triband yagi
> on
> > your tower.  As a shortwave receive antenna , it would work quite well
> > because of the large capture area.  Lots of aluminum up there.  Let's
> say
> > there is a strong sw broadcast station at 16 MHz.  Your tribander
> wouldn't
> > care too much which direction the 16 MHz signal came from because it is
> not
> > directional at 16 MHz.  It could conduct a pretty strong 16 MHz signal
> > towards your radio.   Now consider your new log periodic, which all your
> ham
> > buddies will envy......    It probably has a larger capture area as far
> as
> > lots of aluminum is concerned, because it has more elements.  BUT, it
> will
> > be directional as far as 16 MHz is concerned.  If the 16 MHz signal is
> off
> > to the side or to the rear, there could be much less signal passed
> towards
> > your radio.
> As a long time LP owner/user and being an active SWL, the log captures way
> more signal off the front, side or anywhere compared to any tribander.
> Lots more. I have both.  I've had variety of tribanders - mainly TH-6,
> ATB-34, Classic 33, Classic 36 and anything with traps makes a VERY POOR
> swl antenna. The traps make for a pretty narrow banded system - not a
> surprise.. 19m signals (approx 1 mhz away from the 20m band) are greatly
> attenuated. A 20m monobander (204BA in my case), on the other hand, makes
> a good SWL antenna at 19m.
> The log, once you get out of the ham bands, will continue to have gain
> relatively independent of the frequency, over its design range. I have a
> Sabre 4-30 mhz lp at 92 feet (62.5' boom, 80' elements - loaded below
> about 6 mhz) and it certainly can challenge even a very good high TOI
> front end/radio on a good night, when the 49m,31m and 25m European signals
> are strong. Even from this signal starved too far inland, too far north
> VE6 QTH.  East coast USA would be far worse.  The LP pattern is pretty
> broad, and in some cases this makes for a fine contesting antenna if that
> pattern is desired. But it accepts energy very well on all the other ham
> bands, and if you are m/m or m/s, that isn't going to help your inter band
> QRM situation.  We mainly use mine on 40m, and only till I get more
> monobanders up. But it will continue to get lots of use for general
> purpose HF monitoring and WARC band usage.
> Your concerns are reasonable and likely. Filters, stubs and the like can
> restore selectivity and keep the rcvr front end from being overloaded, so
> the situation isn't that bad, but it will take some effort.
> 73 Don
> VE6JY  Don Moman               email:
> Box 127 Lamont, Alberta        email forwarding:
> (780) 895-2925

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