From: Brendan_Hoar@notes.pw.com
Newsgroups: rec.radio.amateur.misc
Subject: TH78A / Larson / Intermod / mods
Date: 26 Jan 1993 09:58:40 -0600
Organization: UTexas Mail-to-News Gateway
NNTP-Posting-Host: cs.utexas.edu


I recently purchased a Kenwood TH78A.  My guess is that they designed the rig 
specifically with the limited abilities of the rubber duckey in mind.  I 
purchased a Larson 2/70 antenna on the recommendation of a salesman at EEB 
(which is a *very* nice place to visit).  I also purchased a  Larson mag 
mount (for mobile use).  I've been experiencing *serious* intermod and/or 
receiver overload.  I am not an experienced ham (or, to be more precise, not 
a licensed ham), so bear with me if the description of the problem(s) seems 
unnecessary.

Lots of paging tones related interference (sounds like paging tones with lots 
of crackle).  I'm pretty sure it is paging tones and not packet.  Sometimes I 
get it everywhere from 144-148 (receiver overload?) and sometimes just on 
frequencies that are in use (intermodulation distortion?).  In the second 
case, sometimes the intermod is only on a few stations in use, but others are 
perfectly clear.  The frequencies I've been monitoring are FM voice 
repeaters, most of which I pick up crystal clear with the ducky.

My home location is a hilltop in northern VA overlooking (sight blocked by 
trees, thank goodness) a highway (395 north) facing Washington DC.  We are 
about an 8 minute drive or so from the city.  Also about 4-5 minutes from and 
facing the Pentagon (my guess is that they take pains NOT to put out any 
strong signals, though).  Needless to say, the interference is the worst 
there (and I'm inside!).  I haven't tried the Larson in DC, but my guess 
would be that with my current set up, it'd be a bad idea.

I have a few questions.  :)

Since I'm not going to be transmitting until I get my license (tested/passed 
Jan 3), I had assumed that I didn't need a ground plane under the Larson for 
the time being and that ground planes are used only for/mainly for 
transmission purposes.  Am I correct in thinking so?

Has anyone gotten information from Kenwood on reducing the receiver 
sensitivity on the TH78A?  It would be nice if such a thing could be computer 
controlled through the keypad (and hopefully not by a ridiculous power on key 
sequence - see side note below).  I think maybe I will write a letter to 
Kenwood with this request.

Is there such a thing as a dual bandpass filter (array?) that could be placed 
on the feed line that would attenuate any signal coming off the antenna that 
was not in the VHF *and* UHF ham bands?  Or is such a thing a bad idea/not 
available?  If there is such a thing, where can I get one?

Side note:  May I bitch?  Thanks.
  The TH78A is a work of art.  I'm simply amazed.  However, I have a little 
problem with the user interface design.  Why REQUIRE that the rig be powered 
off and then back on with a certain keypress to change certain features?  
Every once in a while, I have to reset the rig (reason is below).  So I power 
up holding down M (to clear memory).  Then I power off.  Then I power up 
holding down 8 (to change scan mode on VHF), then I power off.  Then I power 
up holding down 9 (to change scan mode on UHF), then I power off, etc, etc, 
etc.  This is silly, and may reduce the useful life of the unit.  I have 
jokingly (and sometimes seriously...) thought about buying another TH78A and 
using that as a data library via the clone feature.  However, my bank account 
vetoed that.  Besides, I can't use the clone feature until I can transmit 
anyway.

  The reason I have to reset the darn rig might be my fault I suppose.  Every 
once in a while (maybe once every 50 power ons) I turn it back on and it has 
switched to CHANNEL mode for apparently no reason.  Only set memories are 
available, and they are displayed as the channel number rather than the 
frequency.  There is no way to switch back to VFO mode.  According to the 
manual, 3+power on should switch, but that only produces a beep (The same 
beep every time, indicating that nothing happened).  Note:  I can't get INTO 
channel mode either using 3+power from VFO mode.  Note that I have a newer 
version of the TH78A with D4, D5, D6 disconnected.  It might be a curse that 
Kenwood put into the ROM for those who are following old mod files with new 
radios, but I would hope they wouldn't be so devious.  I could be wrong.  I'm 
waiting for the *new* 5A mod book to show up at my doorstep and maybe I'll 
get more info.  In the end it will probably end up being my fault - but I had 
to get it out.  Now I feel better.  Thanks.  :) :) :)

I too am curious about the beep between every frequency change in the low VHF 
(50+MHZ) range.  Why does it DO that?

Followup either to the newsgroup (which I'll be able to read this weekend 
after my telnet link goes back up - I'm posting thru an email gateway) or to 
badbunny@tfsp.saic.com.  Whichever you prefer is fine with me.

Thanks,
Brendan

PS - Loads of thanks to all of you who answered my earlier query on CW 
software.  I picked up Super Morse 3.16 and have started my journey towards 
the General license.  It might take me a while (since I don't have a PC at 
home), but I'll get there.
Newsgroups: rec.radio.amateur.misc