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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: Don Moman <> (Don Moman)
Date: Sat, 27 Nov 1999 01:14:19 -0700 (MST)
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
> opposite.  Your theory is reasonable but.......  Assume a triband yagi
> 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
> there is a strong sw broadcast station at 16 MHz.  Your tribander
> care too much which direction the 16 MHz signal came from because it is
> 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
> buddies will envy......    It probably has a larger capture area as far
> lots of aluminum is concerned, because it has more elements.  BUT, it
> be directional as far as 16 MHz is concerned.  If the 16 MHz signal is
> to the side or to the rear, there could be much less signal passed
> 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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