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

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Subject: [TowerTalk] Do Log-Periodics Stress the Selectivity of theReceiver?
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Date: Sat, 27 Nov 1999 05:24:43 EST
  The question of a LP antenna overloading receiver is an interesting problem 
as described below.  The argument the LP manufacturer makes 
sounds good but I still find a Yagi has a fair rejection off band by pattern 
and high SWR.  This can be heard switching to off band yagi's while listening 
to strong in band signals which is about the same as having a strong off band 
signal received by an in band antenna.  The LP still amplifies off band 
signals head on and all around lowered by the F/B and with a low SWR.
    The main problem is that all the fancy and extenisve filtering in the 
modern rigs is after the mixer after the damage has been done--it got by the 
mixer.  It's like an ALC circuit in the plate circuit of a linear trying to 
correct the surges of too much drive in the grids.  Smart finals have the ALC 
in the grid circuit like the Henry's and the Ameritron AL-80B. 
     The passive preselector CQ circuit I described in a previous TT is the 
simplest way to enjoy all antennas.  I've been taking a 2nd look at LP's.  
There is an improved version in Compendium 6.  Beams with gains less than 6 
dB gain work best at the lower heights and that 's where I would install it.  
The vertical H plane is wider at the lower gains and really fills the higher 
angle reflection factors of the lower heights--with surprising results.  They 
work very well higher also.  Any antenna needs a selective receiver.  Open 
wire fed antennas using a tuner in the shack have great attenuation off band.
    I think it was a TS-690 mentioned that was a victim of front end over 
load.  I have a TS-680.  On 6M the selectivity is so bad that when I listen 
on 50.125 MHz the call frequency and there is a signal on say 50.170, I can 
hear the products of poor design and it tips me off there is someone else on 
the band without scanning.  So poor spill over selectivity actually serves a 
useful purpose all the time on bands like 6M (or other bands) that close and 
open unpredictably and call frequencies are used.  It's useful on 10M during 
the low sunspot cycles to check for someone on the band somewhere.  I'd like 
a bullet proof receiver to overload signals with a switch that makes it 
equivalent to the TS680 selectivity for monitoring off frequency signals.  In 
the next couple of years as the bands really open and there are far more hams 
on the air, there will be more talk about front end over load of receivers, 
poor AVC circuits, which ones they are and correction ideas.  There is a AVC 
mod for the 1000MP for CW contest Dxers that may help it on SSB also.  I'd 
bet the neon lamps some receivers have across the antenna input light up on 
rigs at a DX expedition.  I know a a case where a DXer pulled a sneaky on 
another DXer by placing a 10 dB filter in his 75A4 receiver coax before a 
contest in LA.  However he heard more weak ones in LA as that much RF that 
close raised the AVC level enough to cover some weak ones.    

<< > 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 >>

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