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"active antenna preamplifiers" in ham radio May 1986

 

Hi All,
?
I found an interesting site, there a archive of the defunct "ham radio" magazine.
?
I have been reading this article intensively for the past few days.
"active antenna preamplifiers by R.W.Burhans, ham radio May 1986" pp.47-54
?
There are things I can't understand.
Please someone tell me what they are?
About "inductive feedback and input capacitance"
that says "A circuit that can reduce the input capacitance and improve the linearity for MOSFETs involves a noiseless feedback method" and "with the FET input capacitance reduced to pratical levels comparable to ..."
Does this mean that the inductance and capacitance cancel each other out?
Or does the NFB current or voltage physically reduce capacitance?
?
About "active notch"?
I have tried to the BC band notch between the Probe and the Amplifier or between the Small Loop and the Amplifier, but it does not work well.
It seems to be difficult with High-Z or Low-Z line.
Putting it in the ?NFB line looks like a good idea.
Have you tried it? Did you get good results?
?
Actually, the MiniWhip I made at the end of last year looks exactly like fig.2 circuit and works well for VLF.
it running at?
?
and I hope to improve it.
I have no electrical or physics education (other than Ham radio), so please forgive my naive question.
?
73, Hisami 7L4IOU
?
 

On Wed, Mar 12, 2025 at 05:28 AM, hisami dejima wrote:
Hi All,
?
I found an interesting site, there a archive of the defunct "ham radio" magazine.
?
I have been reading this article intensively for the past few days.
"active antenna preamplifiers by R.W.Burhans, ham radio May 1986" pp.47-54
?
There are things I can't understand.
Please someone tell me what they are?
About "inductive feedback and input capacitance"
that says "A circuit that can reduce the input capacitance and improve the linearity for MOSFETs involves a noiseless feedback method" and "with the FET input capacitance reduced to pratical levels comparable to ..."
Does this mean that the inductance and capacitance cancel each other out?
Or does the NFB current or voltage physically reduce capacitance?
?
I try to explain it in few words. Yes, inductance and capacitance are canceling each out, this is the fact how the resonant circuits works. In old times, when the tubes have a lots of internal capacitance and feedback, the neutralisation was used and it was based on this principle too. How this work in active circuit is a bit complicated, you have to simulate the circuit and see what happens there. The negative feedback decreases the gain, but? can improve input impedance as well.
?
?
About "active notch"?
I have tried to the BC band notch between the Probe and the Amplifier or between the Small Loop and the Amplifier, but it does not work well.
It seems to be difficult with High-Z or Low-Z line.
Putting it in the ?NFB line looks like a good idea.
Have you tried it? Did you get good results?
?
The graph in the magazine answers clearly to that. Without the feedback, the reflective behavior of the BC bandstop is presenting high impedance on the input, which leads to the peak in gain and sometimes even to instability. The feedback can prevent such problems.
?
?
Actually, the MiniWhip I made at the end of last year looks exactly like fig.2 circuit and works well for VLF.
it running at?
?
and I hope to improve it.
I have no electrical or physics education (other than Ham radio), so please forgive my naive question.
?
73, Hisami 7L4IOU
?
73s, OK1VAW, Vojtech.
?
?
 

Hi Vojtech,
?
Thanks for the reply.
?
"neutralisation" is a nostalgic word.
I hadn't been aware for years.
It filled few pieces of the puzzle.
?
Thanks also for the explanation of the traps.
?
I tried to improvise, but I burned 2N7002 and BS170.
even MOSFET is more clitical than I imagined.
fixed resistance is seems too rough for bias and VR is needed.
?
tnx & 73, Hisami 7L4IOU
?
 

Hi Hisami,
?
The circuit and principles shown in the article are valid, but the reasoning behind them is now somewhat dated. The use of a power FET for this purpose was "state of the Art" at the time of publication, but IMHO has since been superseded.
?
Power FET's tended to present a very high value of input capacitance, which is less than ideal when used with a short? "whip" antenna. The Intermodulation performance of the amplifier in the article is also worse than I would expect to be able to obtain by using more recent devices. Many of the techniques mentioned in the article are attempts to improve these parameters, which are less problematic in modern designs.
?
Unless you are interested in reception at very low frequencies, say <100kHz, it is not necessary to use an amplifier with a very high value of input resistance. By reducing this value, it is possible to relax the design parameters, whilst still being capable of producing good results.
?
On Wed, Mar 12, 2025 at 04:28 AM, hisami dejima wrote:
Please someone tell me what they are?
About "inductive feedback and input capacitance"
?
Yes, you can use feedback to cancel out the opposite value of reactance.
?
?
About "active notch"?
?
?
This is where feedback is used to create a resonant peak, but it is fed back in the opposite phase, to produce a notch in the frequency response
?
Regards,
?
Martin
 

With todays knowledge, the usage of a MOSFETs in the front stage of an active e-field impedance converter is not really a good idea. Even if some professional active-monopoles like the Eastern German KAA1000 with its made-in-Russia KP902A Power-MOSFET went this way in the past. The noise of as MOSFET is much higher compared to a JFET.? You should also avoid a series resistors to the gate or keep it as small as possible, its thermal noise voltage adds to the signal and degrades the noise figure. That 's why i would not recommend the fig.2 circuit in this article.
?
regards
Fred
 

The J-310 would be a much better choice. ??

Dave - W?LEV

On Wed, Mar 12, 2025 at 10:25?AM Fred M via <dl4zao=[email protected]> wrote:
With todays knowledge, the usage of a MOSFETs in the front stage of an active e-field impedance converter is not really a good idea. Even if some professional active-monopoles like the Eastern German KAA1000 with its made-in-Russia KP902A Power-MOSFET went this way in the past. The noise of as MOSFET is much higher compared to a JFET.? You should also avoid a series resistors to the gate or keep it as small as possible, its thermal noise voltage adds to the signal and degrades the noise figure. That 's why i would not recommend the fig.2 circuit in this article.
?
regards
Fred



--
Dave - W?LEV
 

Hi Martin,

Thanks for the advice.
I have some low noise audio FETs (2SK170 etc) that I got a long time ago.
I thought could not use them for RF because of large Ciss.?
However, LTSPICE simulation to show that NFB can be effective.
besides VLF is close to AF, so I will try it.
?
tnx & 73, Hisami 7L4IOU
 

Hi Fred,
?
Thanks for the advice.
?
I had gotten a Soviet KP901B about 20 years ago.
I remembered it and tried to improvise.
?
The radiater is a Jpanese brass coin with a hole.
Indeed, it worked as an Antenna.
The noise level seems to be a little high, almost same as 2N7002.
However, it seems acceptable because of the high noise level in my QTH.
IMD felt it was good.
to make it better, need more acceptance of receiver.

tnx & 73, Hisami 7L4IOU
?
?
?
?
 

Hi Dave,?

Yes, You are right!
There is no need to invest for old devices.
?
tnx & 73, Hisami 7L4IOU
 

Two pictures are attached.

----- Original Message -----
Date: 2025/03/13(Thu) 08:43:55
From: "hisami dejima via groups.io" <7L4IOU@...>
Reply-To: [email protected]
To: [email protected]
Subject: Re: [loopantennas] "active antenna preamplifiers" in ham radio
May 1986


Hi Fred,

Thanks for the advice.

I had gotten a Soviet KP901B about 20 years ago.
I remembered it and tried to improvise.

The radiater is a Jpanese brass coin with a hole.
Indeed, it worked as an Antenna.
The noise level seems to be a little high, almost same as 2N7002.
However, it seems acceptable because of the high noise level in my QTH.
IMD felt it was good.
to make it better, need more acceptance of receiver.

tnx & 73, Hisami 7L4IOU
 

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The AMRAD active antenna is based on Burhan's circuit.??

W1VD also has a version using a common J310 jfet.??

73,

Steve AA7U

On 3/11/2025 9:28 PM, hisami dejima via groups.io wrote:

Hi All,
?
I found an interesting site, there a archive of the defunct "ham radio" magazine.
?
I have been reading this article intensively for the past few days.
"active antenna preamplifiers by R.W.Burhans, ham radio May 1986" pp.47-54
?
There are things I can't understand.
Please someone tell me what they are?
About "inductive feedback and input capacitance"
that says "A circuit that can reduce the input capacitance and improve the linearity for MOSFETs involves a noiseless feedback method" and "with the FET input capacitance reduced to pratical levels comparable to ..."
Does this mean that the inductance and capacitance cancel each other out?
Or does the NFB current or voltage physically reduce capacitance?
?
About "active notch"?
I have tried to the BC band notch between the Probe and the Amplifier or between the Small Loop and the Amplifier, but it does not work well.
It seems to be difficult with High-Z or Low-Z line.
Putting it in the ?NFB line looks like a good idea.
Have you tried it? Did you get good results?
?
Actually, the MiniWhip I made at the end of last year looks exactly like fig.2 circuit and works well for VLF.
it running at?
?
and I hope to improve it.
I have no electrical or physics education (other than Ham radio), so please forgive my naive question.
?
73, Hisami 7L4IOU
?
 

Hi
I have uploaded some pictures to my blog.

I will add circuits and descriptions. If I get enough results to introduce them.?
73, Hisami 7L4IOU
?
 

Hi Hisami,
?
The PA0RDT ?
PA3FWM
PA0NHC dual fet
?
Are probably the best simple designs around at the moment.
?
My favourite remains the Chris Trask (fig.5) which I believe offers the bast trade between performance, cost and ease of construction.
?
?
My previous active antenna experiments can be found at
?
?
Regards,
?
Martin
?
?
On Wed, Mar 12, 2025 at 11:34 PM, hisami dejima wrote:

Thanks for the advice.
 

I have a question about the Trask push-pull amplifier. Doesn't it have a rather low input impedance to be connected to a probe antenna?
?????????????????????????????????????? Mikek
 

Hi ?Mikek,
?
The better IMD performance of the Trask circuit, means that it can be used with a larger antenna (one having greater self capacitance) such as a 1m whip.
?
The coax transmission line forms a major part of the antenna anyway, so I see little point in having an excessively high value of input impedance, just so that a very small "plate" can be used.
?
By using an antenna with a larger value of self capacitance, the issue of the amplifier input shunt capacitance, that otherwise degrades the Signal to Noise performance, is minimised.
?
If you use a small plate with a self capacitance value of say 0.5pF, and the amplifier input capacitance is say 2pF (including mounting), then the majority of the input signal is shunted to ground. A 1m whip has a value of self capacitance of around 10 - 15pF, so the amplifier shunt capacitance is much less of an issue.
?
Using a larger antenna, also reduces the need for an excessively high value of amplifier input resistance, which also helps reduce problems associated with choking common mode noise induced in the feed line.
?
Regards,
?
Martin
?
On Thu, Mar 13, 2025 at 03:42 PM, Mikek wrote:

I have a question about the Trask push-pull amplifier. Doesn't it have a rather low input impedance to be connected to a probe antenna?
 

Hi Martin,

Thanks for the infomation and advices.
Thanks also for the recommend on "Chris Trask (fig.5)".
I will look for the FETs.
I have several pairs, but they all seem to have large Ciss.

tnx & 73, Hisami 7L4IOU
 

Hi Hisami,
?
This is the paper the design was taken from.
?
?
Regards,
?
Martin
?
On Thu, Mar 13, 2025 at 11:27 PM, hisami dejima wrote:

Thanks also for the recommend on "Chris Trask (fig.5)".
 

A tour de force by Chris Trask!!

Re my previous suggestion of using constant current sources replacing R1 and R2 in Everett's great circuit, I leave the following quote by Chris....

>> The first of these circuits is shown in Fig.4. Here, the source load inductor of the KAA 1000 has been replaced with a JFET constant current source (Q2), where resistor R3 determines the bias current for the JFET source for Fig. 5 - Complementary Push-Pull Amplifier with Complementary Push-Pull Input Stage
lower (Q1). This active load provides a very high load impedance for the Q1 source follower, which in turn results in better IMD performance of the input stage.<<

best wishes

Paul VE3PVB
Collingwood ON CANADA
On Friday, March 14, 2025 at 06:30:10 a.m. EDT, Martin - Southwest UK via groups.io <martin_ehrenfried@...> wrote:


Hi Hisami,
?
This is the paper the design was taken from.
?
?
Regards,
?
Martin
?
On Thu, Mar 13, 2025 at 11:27 PM, hisami dejima wrote:

Thanks also for the recommend on "Chris Trask (fig.5)".
 

On Thu, Mar 13, 2025 at 08:54 AM, Martin - Southwest UK wrote:
The better IMD performance of the Trask circuit, means that it can be used with a larger antenna (one having greater self capacitance) such as a 1m whip.
?Hi Martin, Thank you very much for the explanation, it all makes sense. Yes I was thinking flat plate antenna.
???????????????????????????????????????????????????????????????????????? Mikek
 

Ralph Burhans wrote an excellent series of antenna articles in Radio Electronics in 1983:
? ?February 1983, part 1 - All about VLF Active Antennas
? ?March 1983, part 2 - All about VLF Active Antennas,?plus corrections, August 1983, p. 25
? ?April 1983, part 3 - VLF-HF Active Antennas
? ?May 1983, part 4 -?VLF-HF Passive Antenna Tuner
? ?June 1983, part 5 - Low Frequency Loop Antennas

In addition he contributed these articles to Radio Electronics in 1983:
? ?July 1983 - Using Loran for Time and Frequency Calibration
? ?September 1983 - Frequency Calibration using WWV
? ?December 1983 - Interference Traps for SWL's

At Fig. 7 of his 1986 article in Ham Radio Magazine, Burhans showed an VLF-LF utility amplifier using a noiseless feedback input.? Ulrich Rohde had shown these techniques in his article in the November 1979 issue of Ham Radio Magazine.

The AMRAD active antenna referenced by Steve Ratzlaff below used a CP666 FET from Crystalonics Inc..? They seem to have closed or moved in the last few months.? Unfortunately their web site
? ??
is no longer working.? Their telephone number is no longer in service.??

73? John? KC0G/M0KCY


On Wednesday, March 12, 2025 at 10:34:35 PM CDT, Steve Ratzlaff via groups.io <ratzlaffsteve@...> wrote:


The AMRAD active antenna is based on Burhan's circuit.??

W1VD also has a version using a common J310 jfet.??

73,

Steve AA7U

On 3/11/2025 9:28 PM, hisami dejima via groups.io wrote:
Hi All,
?
I found an interesting site, there a archive of the defunct "ham radio" magazine.
?
I have been reading this article intensively for the past few days.
"active antenna preamplifiers by R.W.Burhans, ham radio May 1986" pp.47-54
?
There are things I can't understand.
Please someone tell me what they are?
About "inductive feedback and input capacitance"
that says "A circuit that can reduce the input capacitance and improve the linearity for MOSFETs involves a noiseless feedback method" and "with the FET input capacitance reduced to pratical levels comparable to ..."
Does this mean that the inductance and capacitance cancel each other out?
Or does the NFB current or voltage physically reduce capacitance?
?
About "active notch"?
I have tried to the BC band notch between the Probe and the Amplifier or between the Small Loop and the Amplifier, but it does not work well.
It seems to be difficult with High-Z or Low-Z line.
Putting it in the ?NFB line looks like a good idea.
Have you tried it? Did you get good results?
?
Actually, the MiniWhip I made at the end of last year looks exactly like fig.2 circuit and works well for VLF.
it running at?
?
and I hope to improve it.
I have no electrical or physics education (other than Ham radio), so please forgive my naive question.
?
73, Hisami 7L4IOU
?

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