[AMRadio] Pi-Net vs Link Couple


Mike Dorworth,K4XM k4xm at arczip.com
Wed Sep 21 23:10:59 EDT 2005


The Pi-Net will give a total of 50 db suppresion of harmonics. about 20 in
one spot and 30 in the other. Link couple can pass harmonics to VHF by
capacity coupling, hence the Faraday Shield Links used for same later on.
There is a nice single 450th Pi-Net rig shown in the Editor and Engineers
handbook. Also a couple of single ended ones in the 1950 ARRL Handbook (for
triodes).  Some triodes that require lots and lots or drive can unbalance
the grid tank, which is required for triodes using Pi-Net. Lo capacity tubes
like the 450th is OK. The old timers mostly used tuners (antenna) and open
wire feeders to keep the harmonics down. Hazletine link neutralization can
also be used and no split tanks are needed in or out. Remember Class C ,
which is required for Hi level AM, is a extreme distortion  and harmonic
generator so that some plan need to be in place to handle the soup. Also a
single band dipole is very frequency selective and cuts way down on
harmonics by itself. Multiband dipoles, beams and multi dipole on one feeder
and traps etc (G5RV) are an open invitation to spread gook with only link
output. Also the guys that use CB lin years with no half wave filters get
away in mobile service without too many problems  due to the narrow
frequency discrimination of mobile antennas. Hope this helps, 73 Mike



----- Original Message ----- 
From: "Geoff" <w5omr at satx.rr.com>
To: "Discussion of AM Radio" <amradio at mailman.qth.net>
Sent: Wednesday, September 21, 2005 10:45 PM
Subject: [AMRadio] Pi-Net vs Link Couple


> SO, here we are, well past 1991 and the 'law' that went into effect
> saying that 1,500w PEP output is the maximum RF Power output that we
> hams can run, regarldless of mode.  That doesn't deter the homebrewing
> spirit, but it does suggest that acheiving 1,500w PEP output is much
> easier than producing 1kW DC input to the final.  With the mindset of
> still wanting to use the classic high-level plate modulation scheme,
> engineering a rig to use only one tube in the final (a 4-250, 250TH,
> 304TH/TL, 4-400, etc), modulated by a pair seems to make more common
> sense.  That, and it's a bit more economic in filament requirements.
>
> I've heard recently that matching the output of the Class C
> pate-modulated final to the antenna is better, and more efficiently
> achieved by link coupling, vs Pi-Net.  On the other hand, it's argued
> that Pi-Net coupling produces less RFI than link coupling does.
>
> So, which is better?
>
> Why?
>
> What are the effects of nuetralizing a single tube in a balanced tank
> circuit?  If Pi-Net is to be used, does the final tube still need to be
> nuetralized?
>
> I know of a guy who wants to build a rig using a single 450TL in the
> final, modulated by a pair.  He wants to pi-net the output, but I've
> heard that's a bad idea.
>
> I want to build a rig using a medium powered tride, perhaps a 250TH,
> modulated by a pair of 811's.  Pi-Net, or Link Couple?
>
> I like seeing this kind of technical discussion on the list.  I'm
> looking forward to all inputs.
>
> --
> 73 = Best Regards,
> -Geoff/W5OMR
>
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