I have one of JC's Waller Flags on a 40 foot boom at 95 feet.. It is fed
with LMR-600 at the tower base XFMR where the 100 ohm twin-ax
terminates. It is use with a K-3 in diversity receive with a Hi-Z 8
which is down the hill es across the creek. The system hears remarkably
well.. Beverages, Ewes, and other RX antennas are out the door...
In the shack the 40 dB N4IS pre-amp is in a steel box for shielding
purposes. All components of the station are connected via 1/2 " braid
to a 6 ' x 6" x 1\4' aluminum grounding bus.
There are multiple chokes in the leads per N4IS es NX4D... It has been a
long process.
The IC-7610 is also in use from time to time since this SDR radio is
Ultra-Quiet...
73,
John, W4NU
On 11/15/2018 7:19 AM, JC wrote:
Hi Jim and Lee
Before we agree that we disagree, let me elaborate on few basic concepts for a
good design. Point by point and let me know which one you disagree.
1- RF runs outside the cable surface, it does not matter what is inside, a coax
cable shield, a solid # 4 wire external surface is similar to a RG58 in respect
of RF current.
2- Every cable on your station is an antenna. If the cable is 1/8 to 1/2 wave
long on low bands, the energy on the outside surface is very high. 100ft rotor
cable , or 100ft 9913 from your 2m antenna, or 100ft of controls cable, and or
100ft of RG6 on your RX antenna have almost the same energy of your 160m
inverted L ~ 120ft.
3- All these cables somehow are connected to your station ground at your
station. All of them are part of your antenna system and interact with each
other.
4- Any of these cables connecting into a well-designed board brings a lot of energy on
low bands, normally called common mode noise, signal that we don’t want to mix
with our RX signal coming from our RX antenna.
5- Prevent the external RF current to enter into our board is a big problem on
low bands. On Audio, you have an excellent description of pin 1 problem on your
papers, 60 and 120 Hz is the issue. On low bands 1.8 MHz, all RF signals from
50 KHz to 10 MHz are responsible for the common mode noise current on low band
antennas.
6- To filter or decouple 1.8 MHz signal a 1000 pf or 1nF has a very high
impedance, 10nF is not enough, it is necessary 100 nF or more. DC filter is an
issue too, it is easy to inject the common mode noise into the Vcc.
7- May point is that is very difficult to protect any board or parts, like a
BALUN or transformer, or any amplifier from common mode noise, PIN 1 PROBLEM.
A plastic box make almost impossible to avoid that. A Metal case protects the
board and avoid the external current to get into the board.
8- I can agree that the intensity of the signal and the common mode signal
leak could be 20 db, 30 db or more. However when you dig a weak signal it is
huge problem.
73'
JC
N4IS
-----Original Message-----
From: Jim Brown [mailto:jim@audiosystemsgroup.com]
Sent: Wednesday, November 14, 2018 8:10 PM
To: n4is@n4is.com
Cc: lee@k7tjr.com
Subject: Re: Topband: 40m array as RX antenna
On 11/14/2018 4:41 PM, n4is@n4is.com wrote:
I would suggest a metal box to protect any RX system, it does help.
Only if the circuit layout is poor. Lee is right - shielding of circuity is
only a band-aid for poor design.
73, Jim K9YC
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