Topband: 40m array as RX antenna
k7tjr at msn.com
Thu Nov 15 17:40:57 EST 2018
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...
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.
From: Jim Brown [mailto:jim at audiosystemsgroup.com]
Sent: Wednesday, November 14, 2018 8:10 PM
To: n4is at n4is.com
Cc: lee at k7tjr.com
Subject: Re: Topband: 40m array as RX antenna
On 11/14/2018 4:41 PM, n4is at 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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