Hello JC,
There is nothing to disagree about JC, My 20 to 24 foot elements when fed
into a Hi-Z amp produce a signal that is around 20 dB less than a good TX
vertical. Your Waller flags produce a signal that is 40 to 50 dB below a TX
vertical. What I said was that because of this difference in absolute signal
level, Hi-Z systems could survive just fine in plastic boxes. Any other lower
gain element certainly needs more protection like metal shielding, you are
quite correct. I have a system I have been working on for years that falls in
this same category of small signals that require more shielding. I do
understand. Digging a weak signal is not a huge problem when you have 20 dB
more signal to work with.
I do not dispute what you say in regard to your Waller flag antennas. Do the
plastic boxes let extraneous signal in? Yes but not enough to disturb the Hi-Z
systems. And yes, circuit design is very important in both these cases. And
yes, Hi-Z systems have internal common mode protection on various in and out
connectors. They also have protection From signal injection from VCC. Plus a
great deal more.
I have no issue with what you say. I do not disparage your systems. I just
don’t need the hundreds of Hi-Z customers calling about changing to metal boxes
on my systems that won't be improved by this type of change...
73
Lee K7TJR
Hi-Z Antennas
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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