Re: [TenTec] SDR’s, You and RFI - Real Long

[jerome schatten]

John:  Your fine work notwithstanding, this issue begs the question: Why 
should SDR's be any more sensitive to RFI than any other radio that has 
microprocessors, lsi chips, memory, dsp's, etc,. under the hood?  The answer 
IMHO, is that they shouldn't.  More careful attention to the layout, 
shielding, cabling and bypassing (good mechanical and electronic 
construction techniques--hardly rocket science) would have produced a radio 
that didn't have the RFI problem in the first place.

The answer: "..Welcome to the world of SDR's" doesn't address the problem at 
the source, the design stage.  One should not have to rewire the 
neighborhood to make a radio work properly.

My comments are not meant to diminish in any way the wise and valuable 
suggestions you've made as regards grounding and RFI minimization -- my only 
point is that it could have been done under the hood with less ruckus.  If 
we except the 'inevitability' that SDR's will always be problematic 
performers in high RF fields, we're voting for more of the same.

Best regards,
Jerome - VA7VV

----- Original Message ----- 
From: <N0KHQ at aol.com>
To: <tentec at contesting.com>; <orion at contesting.com>
Sent: Friday, December 10, 2004 2:46 PM
Subject: [TenTec] SDR’s, You and RFI - Real Long


I am posting this once again for those having RFI problems and believing
that it is a problem with their transceiver.

I apologize for the lengthiness.


SDR’s, You and RFI

I have spent the last three years investigating  "Special Grounding"
techniques.

What I'm going to suggest here is a look  at your entire grounding system.
I'm talking about RF, AC, Telephone, Cable TV  grounding and DC grounding.

AC Grounding:
1. All 115v power for your  HF/VHF equipment should come from one 
receptacle.
Failure
to do this may  lead to ground loops via the green wire that runs through 
out
your home.
This receptacle should not shared with other appliances in your home. The
receptacle I use is a dedicated circuit for the clothes washing machine.

2. Take a look at your electrical service panel. There is a very heavy 
gauge
wire running
from the ground bus outdoors to an 8' copper grounding  rod. Hopefully this
wire is copper
and not aluminum. If it is aluminum have  it replaced with copper.

Assuming that the grounding wire is copper,  disconnect the ground wire,
clean it and
the grounding rod with a wire brush  and/or sand paper. Reconnect the wire 
to
the
grounding rod with a suitable  type clamp. I use a very large split-bolt.
Tape it and
make it water tight.

You will also note that the telephone people and the cable people may  have
terminated
their equipment signal ground on your AC ground. Install a  separate
grounding rod for
this equipment, tape it and make water tight.  These are special purpose
grounds and should be treated separately.

Note: The NEC is concerned with grounding for safety only. They could  care
less about RF.

This next topic is my favorite, RF Grounding. First a  little history.

My antenna, a Sterba Curtain, is directly (about 10')  over the top of my
home. On 160m, 75m and 40m I run a full 1000 watts with no  RFI or TVI. I 
also
use a ton of sound reinforcement equipment at my HF operating  position 
also.


Your station HF/VHF station should have what is call a  "single point" RF
ground. I'll explain.
Lets say that your RF chain is as  follows:

1. Transceiver
2  Amplifier
3. Antenna Tuner
4. RF Grounding Rod
Under the above conditions, there should only be one  wire going from the
grounding lug on the back of the antenna tuner out to the  grounding rod. Do 
not
ground each piece of equipment to a central point and then  out to the
grounding rod. Each piece of equipment is already RF grounded through  the 
inter
connecting equipment jumper coax shields. Failure to ignore the above  may 
result
in ground loops.

The RF ground wire I use is a length of  RG-8. Only the center conductor is
terminated at the TT-1251 tuner. Outdoors at  my RF grounding tie point the
center conductor is shorted to the shield. this  will effectively drain any 
RF to
the Star RF Grounding Counterpoise  System.


If your antenna is a multi-band dipole. A dipole is a  balanced antenna, if
you are feeding the dipole with unbalanced coax the shield  of your coax, 
and
your entire station, is acting as the other half of the  antenna system.
Installation of a 1:1 Balanced to Unbalanced Current Balun at  the antenna 
feed
point will stop return currents on the coax shield and  gracefully make the
transition from a balanced antenna to an unbalanced system.  If you are 
running a
vertical antenna install a Current Balun Kit (as sold by 
Polomar-Engineering)
at the feed point.

>From Radio Works, purchase  a 50ohm bal to 50ohm unbal Current Balun (B1-5K
cost $34). This device installs  at the feed point of the dipole. Also 
purchase
a T-4 Line Isolator, this device  installs between your rig and the
amplifier.

I recommend the following  RF chain:

1. Transceiver  (Orion, TS-950SDX, IC-756 Proll)
2.  T-4 Line Isolator (from Radio Works)
3. Amplifier (AL-80B) Don’t forget to  put a toroid on the keying line.
4. Tuner (TT-238B)
5.  TT-1251(Counterpoise RF tuner from Ten-Tec)
6. Star RF Grounding system  (detailed construction drawings are available
upon request)

Note: An RF  Grounding system will help with lightning protection, but a
Grounding system for  lightning protection will help very little with RF
Grounding. Don’t confuse the  two.

I have probably furnished more information than you want. But, to  address
some issues with SDR’s and unusual operations, it was necessary.

I can hear it now……guys saying “ I never had to do that with my old  rig…
..what a bunch of BS”………. Well…. welcome to the world of SDR’s.
,

Have fun


John / N0KHQ / St. Louis

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