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Re: Topband: 40m array as RX antenna

To: <jim@audiosystemsgroup.com>, <topband@contesting.com>
Subject: Re: Topband: 40m array as RX antenna
From: "JC" <n4is@n4is.com>
Date: Thu, 15 Nov 2018 07:19:24 -0500
List-post: <mailto:topband@contesting.com>
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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