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Modified AMRAD

 

I made the corrections that Martin mentioned and I also add the Bias adjustment procedure to the posting and deleted the 2 original postings.
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Everett N4CY
 

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That will teach me for DL'ing so quickly LOL.....can you post a URL to the corrected document? 73
Gedas, W8BYA EN70JT
Light travels faster than sound.....
This is why some people appear bright until you hear them speak.
On 3/20/2025 2:30 PM, Everett N4CY via groups.io wrote:

I made the corrections that Martin mentioned and I also add the Bias adjustment procedure to the posting and deleted the 2 original postings.
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Everett N4CY
 

/g/loopantennas/files/Modified%20AMRAD%20eProbe%20Antenna.pdf

In a message dated 3/20/2025 1:42:59 PM Central Daylight Time, w8bya@... writes:
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That will teach me for DL'ing so quickly LOL.....can you post a URL to the corrected document? 73
Gedas, W8BYA EN70JT
Light travels faster than sound.....
This is why some people appear bright until you hear them speak.
On 3/20/2025 2:30 PM, Everett N4CY via groups.io wrote:
I made the corrections that Martin mentioned and I also add the Bias adjustment procedure to the posting and deleted the 2 original postings.
?
Everett N4CY
 
Edited

I doubt, that paralleling FETs in an reactance driven HiZ? application is worth the effort. The noise figure of an Hi-Z FET Amplifier ist primarily dominated by the thermal noise of the Gate bias Resistor of 22 Megohm which is much higher, than the FET noise. It may make sense in Low-Noise Audio applications, where the source impedance is low, but IMHO not as an e-field probe for RF-reception.
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For the use as an short monopole e-field antenna, the common mode choke at the output is "stealing" the earth/ground reference, which is normally provided through the braid of the coaxial cable and which is necessary to pick up the voltage? (U = E x heff) induced in the probe by an electric field. With an CMC at the output, a separate earth connection to the amplifier ground is mandatory, otherwise the amplifier cannot "measure" the potential-difference between probe and earth, because it lacks a ground reference.
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More reading:
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regards
Fred
 

I built this circuit and used it for many years. In fact is was my first antenna for NDB DX 10 years ago. I started with 1 FET and experienced some IMD from local AMBCB stations. Then I added 3 more FETs to increase the current. This reduced IMD noticeably. So the benefit in my opinion is not noise reduction, but large signal handling.
73 Tom
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I would agree.
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However, I seem to remember reading that parallelling FET's can improve the overall noise performance, but can't find the reference to quote.
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It does however reduce the input impedance. This is not too much of an issue, as many E-Field amplifier circuits have far too high a value anyway, but there is a compromise to be reached in terms of the number of FETs used in a practical implementation.
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Regards,
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Martin
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On Fri, Mar 21, 2025 at 12:23 PM, Tom - VE3PSZ wrote:

So the benefit in my opinion is not noise reduction, but large signal handling.
 

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Yes parallel JFETs can lower the noise at the signal gain is linear and the noise being random ?adds as root 2 or 41.4% for each doubling. Eventually the gate capacitance will be an issue.
Andrew VK5CV.

Get

From: [email protected] <[email protected]> on behalf of Martin - Southwest UK via groups.io <martin_ehrenfried@...>
Sent: Friday, March 21, 2025 10:59:26 PM
To: [email protected] <[email protected]>
Subject: Re: [loopantennas] Modified AMRAD
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I would agree.
?
However, I seem to remember reading that parallelling FET's can improve the overall noise performance, but can't find the reference to quote.
?
It does however reduce the input impedance. This is not too much of an issue, as many E-Field amplifier circuits have far too high a value anyway, but there is a compromise to be reached in terms of the number of FETs used in a practical implementation.
?
Regards,
?
Martin
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On Fri, Mar 21, 2025 at 12:23 PM, Tom - VE3PSZ wrote:
So the benefit in my opinion is not noise reduction, but large signal handling.
 

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?? There is a reference here to Andy Ikin's paralleling of FETs, lowering the noise figure.
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??
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???The antenna uses 8 very high gain JFETs in parallel push-pull with a Bipolar transistor cascode stage for extended bandwidth.
?? A very low amplifier noise floor is achieved by dynamically decreasing the JFET Source resistance to a fraction of an Ohm.
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? Regards
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??Tracey
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----- Original Message -----
From: Martin - Southwest UK via groups.io <martin_ehrenfried@...>
Reply-To: <[email protected]>
Sent: 21/03/2025 12:29:26
Subject: Re: [loopantennas] Modified AMRAD
I would agree.
?
However, I seem to remember reading that parallelling FET's can improve the overall noise performance, but can't find the reference to quote.
?
It does however reduce the input impedance. This is not too much of an issue, as many E-Field amplifier circuits have far too high a value anyway, but there is a compromise to be reached in terms of the number of FETs used in a practical implementation.
?
Regards,
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Martin
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On Fri, Mar 21, 2025 at 12:23 PM, Tom - VE3PSZ wrote:
So the benefit in my opinion is not noise reduction, but large signal handling.
 

On Fri, Mar 21, 2025 at 01:04 PM, Tracey Gardner G5VU wrote:
The antenna uses 8 very high gain JFETs in parallel push-pull with a Bipolar transistor cascode stage for extended bandwidth.
Yes i know, but this is comparing apples with oranges. The Ikin Amp is a balanced very low Z Input amplifier for magnetic loops. The current discussion is about Hi-Z impedance change amplifiers for short Monopole Antennas aka active Whips.
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regards, Fred
 
Edited

On Fri, Mar 21, 2025 at 12:23 PM, Tom - VE3PSZ wrote:
This reduced IMD noticeably. So the benefit in my opinion is not noise reduction, but large signal handling.
A better way to improve large-signal handling is to replace the source resistor of the input FET with an active current source. This increases linearity and also reduces the gain loss of the source follower. I've done that with all my E-field Whip amplier designs (see link below) . Paralleling FETs is IMHO counterproductive, because you increase the input capacitance by the number of FETs. This input capacitance forms a cpacitive voltage divider with the capacitance of the Whip, which can be assumed as between 5 and 10 pF. With 10 pF antenna capacitance and 20 pF input capacitance of 4 x FETs, you get only 1/3rd of the EMF input voltage from the field probe. In combination with the negative gain of s source follower and the emitter follower you end up with an overall gain of -10 dB. These 10 dB have to be added to the inherently bad noise figure of an Hi-Z FET amplifier.
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I've uploaded two of my active-whip amplifiers. Many of them have been built and they perform greatly.
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regards
Fred
 

Hi Fred,
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I've had a look at your circuit diagrams, and wondered if you could clarify a point for me.
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I'd assumed that placing an active current source, in place of the source resistor, would set the FET bias current. But you also include a gate bias voltage adjustment. How do these two interact ?
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I'd also have thought that the characteristics of the FET, and value of the source resistor, would have pretty much defined its bias point, regardless of any additional current source or voltage bias, but maybe I'm misunderstanding the architecture of FET circuits.
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Interesting stuff.
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Regards,
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Martin
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On Fri, Mar 21, 2025 at 01:28 PM, Fred M wrote:

A better way to improve large-signal handling is to replace the source resistor of the input FET with an active current source. This increases linearity and also reduces the gain loss of the source follower. I've done that with all my E-field Whip amplier designs (see link below)

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