?? 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
?
??Tracey
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----- Original Message -----
Sent: 21/03/2025 12:29:26
Subject: Re: [loopantennas] Modified AMRAD
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.
?
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
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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
?
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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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
?
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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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 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
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File /MODIFIED AMRAD PREAMP - 20 MARCH 2025.docx uploaded
#file-notice
The following items have been added to the Files area of the [email protected] group.
By: W0LEV <davearea51a@...>
Description:
Build and technical evaluation with measurements of the Modified AMRAD high-Z input preamp for E-Field probes. Dave - W?LEV
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/g/loopantennas/files/Modified%20AMRAD%20eProbe%20Antenna.pdf
In a message dated 3/20/2025 1:42:59 PM Central Daylight Time, w8bya@... writes:
?
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
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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:
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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
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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
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Re: "active antenna preamplifiers" in ham radio May 1986
Thanks Martin, I will correct it.
On Thursday, March 20, 2025, 11:45 AM, Martin - Southwest UK via groups.io <martin_ehrenfried@...> wrote: Hi Everett,
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I think if you do this, you will have to spell out that a separate "ground" reference for the antenna and PCB needs to be provided, as the coax performs this function by default in most other designs.
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I know it seems obvious, but many end users don't understand this significant point.
?
Regards,
?
Martin
?
?
On Wed, Mar 19, 2025 at 06:24 PM, Everett N4CY wrote:
I designed a new board that has a common mode choke on the back side of?the board,
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Re: "active antenna preamplifiers" in ham radio May 1986
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So we can not do attachments OR embedded graphics in the body of an email??? A picture is worth 1k words.? But I understand the cost when archived in storage.? These days, storage is the least expensive of digital options.? But I still understand.?
Today I will be attempting to make a frequency sweep of my unit.? I'll wait to upload to the files until that is complete. ?
Dave - W?LEV
On Thu, Mar 20, 2025 at 8:06?AM Martin - Southwest UK via <martin_ehrenfried= [email protected]> wrote: Yes, that's the default setting in this group, attachments are removed.
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Dave, can you add it in the files section please.
?
Regards,
?
Martin
?
?
?
On Thu, Mar 20, 2025 at 06:47 AM, Andrea Dalbagno wrote:
It seems that the attachment has been removed from your email.
--
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Re: "active antenna preamplifiers" in ham radio May 1986
Hi Everett,
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Incidentally, note that in your schematic, one of the input diodes, seems to have the wrong polarity.
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Personally, I'd bias them to ensure decent IMD performance, but maybe you have already measured this using a high Z source and considered it to be OK ?
?
Regards,
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Martin
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Re: "active antenna preamplifiers" in ham radio May 1986
Hi Everett,
?
I think if you do this, you will have to spell out that a separate "ground" reference for the antenna and PCB needs to be provided, as the coax performs this function by default in most other designs.
?
I know it seems obvious, but many end users don't understand this significant point.
?
Regards,
?
Martin
?
?
On Wed, Mar 19, 2025 at 06:24 PM, Everett N4CY wrote:
I designed a new board that has a common mode choke on the back side of?the board,
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Re: "active antenna preamplifiers" in ham radio May 1986
Hi Dave,
?
The free versions of IO groups, have limitations on storage. These limitations depend on the T&C's of when the group was established, the older groups have more free storage than younger groups. It was offered as an incentive to early adopters.
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If folks include images or files in the body of their posts, it automatically gets included and stored, which counts towards the overall limit.
?
IO also reduces the resolution of images included in posts, but not when uploaded to images, and any messages that include previous posts with files or images can end up being copied multiple times.
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By getting folks to deliberately upload to the files and photos area, only items that are likely to be of long term use get to be stored, and the more trivial items are excluded.
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I hope this helps provide a bit of the background as to why the rules on various groups sometimes differ.
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Regards,
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Martin
?
On Thu, Mar 20, 2025 at 04:12 PM, W0LEV wrote:
So we can not do attachments OR embedded graphics in the body of an email???
|
Re: "active antenna preamplifiers" in ham radio May 1986
So we can not do attachments OR embedded graphics in the body of an email??? A picture is worth 1k words.? But I understand the cost when archived in storage.? These days, storage is the least expensive of digital options.? But I still understand.?
Today I will be attempting to make a frequency sweep of my unit.? I'll wait to upload to the files until that is complete. ?
Dave - W?LEV
On Thu, Mar 20, 2025 at 8:06?AM Martin - Southwest UK via <martin_ehrenfried= [email protected]> wrote:
Yes, that's the default setting in this group, attachments are removed.
?
Dave, can you add it in the files section please.
?
Regards,
?
Martin
?
?
?
On Thu, Mar 20, 2025 at 06:47 AM, Andrea Dalbagno wrote:
It seems that the attachment has been removed from your email.
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File /Modified AMRAD eProbe Antenna.pdf uploaded
#file-notice
The following items have been added to the Files area of the [email protected] group.
By: Everett N4CY <everettsharp@...>
Description:
Active eProbe antenna
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Re: "active antenna preamplifiers" in ham radio May 1986
Mike,
?
I just uploaded it.
?
Everett N4CY
In a message dated 3/20/2025 10:44:13 AM Central Daylight Time, amdx@... writes:
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Has the 4 FET input High Z amp schematic been posted to the files section, if so under what heading?
??????????????????? Thanks, Mikek
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Re: "active antenna preamplifiers" in ham radio May 1986
Has the 4 FET input High Z amp schematic been posted to the files section, if so under what heading?
??????????????????? Thanks, Mikek
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Re: "active antenna preamplifiers" in ham radio May 1986
Hi Henry,
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I just quickly built one up.
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The gain seems quite low and the IMD performance isn't that good. Current draw is about 10mA, which is quite low.
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I need to experiment to see if I can further optimise the circuit, but it's not looking too promising.
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When folks state things like "good strong signal handling" without providing any figures, or other parameters as evidence, it's difficult to know if a circuit is performing as expected.
?
Regards,
?
Martin
?
?
On Wed, Mar 19, 2025 at 02:30 AM, <tardivat@...> wrote:
Have you tried the G4COL design with? transformer in the FET circuit?
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Everett N4CY