Hi Bill
On what frequencies?
Simon g0zen
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Re: "active antenna preamplifiers" in ham radio May 1986
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
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Re: "active antenna preamplifiers" in ham radio May 1986
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)".
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Re: "active antenna preamplifiers" in ham radio May 1986
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)".
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Hi Bill,
I suggest you model your exact antenna and radial dimensions in eznec and check f/b at 2 degrees. Try optimising the f/b with a resistor 100-600 ohms in series with insulated radials on their own. You can add the other structures to see their effect.?
73
John
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Re: "active antenna preamplifiers" in ham radio May 1986
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
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I have just installed an Aziloop and I do not get good nulls - 10 -15 dB on 50 -100-mile ground wave signals,
Loop is 13' on long side, 8' on short side and PVC mast is 15 feet. Radials are on the ground below the loop elements. It is fed with about 200' of RG6 cable and the?supply voltage is 12.7 volts.
I have a SAL-20 antenna 40 feet north of the AZI.? I can get 15-30 dB nulls with it.?
I have a 60 foot self-supported welded steel tower with a large 10–40-meter Mosley beam at the top 42' east of the Azi.
Antennas are in the middle of a 1.5 acre lot - no other buildings or metal nearby.? Ground elevation is 15 feet ASL.
Could these two other antennas be affecting the poor null operation on the Azi???
Any suggestions would be appreciated.
73,
Bill, WA2DVU Cape May, NJ 609 425 8651
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Re: "active antenna preamplifiers" in ham radio May 1986
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?
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Re: "active antenna preamplifiers" in ham radio May 1986
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
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Re: "active antenna preamplifiers" in ham radio May 1986
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:
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Re: "active antenna preamplifiers" in ham radio May 1986
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
?
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Re: "active antenna preamplifiers" in ham radio May 1986
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:
toggle quoted message
Show quoted text
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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Re: "active antenna preamplifiers" in ham radio May 1986
Two pictures are attached.
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----- 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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Re: "active antenna preamplifiers" in ham radio May 1986
Hi Dave,?
Yes, You are right!
There is no need to invest for old devices.
?
tnx & 73, Hisami 7L4IOU
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Re: "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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Re: "active antenna preamplifiers" in ham radio May 1986
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
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Re: Reasonable size of an AziLoop?
I can hardly rotate a 200m wire - !!? ;-) That reminds me of the note in the AziLoop manual, “To be clear, the antenna does not move” Thanks for your explanation and assessment. Especially the reference to the earthing, which I will lay cleanly underground because of the animals. And you also mention the “small” size, so the 13.8 meter circumference mentioned by Dave and Guy with a solid grounding will be a good solution for me and my question here has been answered well. Of course, the comparison with my 200 meter Beverage will be interesting, but the main reason for the installation is simply the flexibility of this system in all directions, which allows the advantages of K9AY and Loop in an actually very small space, and everything can be controlled via IP. This is already a very good job by Dave, who will soon be releasing a software update with some new features and improvements, and in the new guide he will also provide further antenna suggestions. Now I'm thinking about how best to erect a self-supporting structure. Given the relatively small size, additional guy ropes may not even be necessary. Thanks again John for your help. 73 Christoph, OE2CRM —
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Re: Reasonable size of an AziLoop?
I currently have an Azi-Loop in the construction process; Thanks Guy, I know your and Walt's ambitions for AM-DX, your solution is already very perfect. I can see that the size (one meter up or down) will be a very good option, also the self-supporting structure is a perfect solution, especially because of animals. I'm already looking forward to a short report on the antenna from your upcoming DXpedtion. My plan is to erect the antenna in the summer, just in time before the first Asian signals can be heard in Central Europe in the fall. Thanks also for the photo, very helpful. Good luck with the AziLoop on your DXpedtion! 73 Christoph, OE2CRM —
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Re: "active antenna preamplifiers" in ham radio May 1986
The J-310 would be a much better choice. ??
Dave - W?LEV
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
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Re: Reasonable size of an AziLoop?
Hi Christoph,
I use 16m (50ft) circumference (W=20ft, top H=15ft, bottom H=4ft) .? I decided on this size for 9dB or 10dB noise over-ride from the antenna on 160m. using only 4 x 3.5m elevated radials (H=3ft) as a counterpoise.
In K9AY loop mode, the antenna works very well on MW as well as 160m and 80m., with measured groundwave f/b in the region of 30dB, degrading considerably on 7 and 10MHz..
The sensitivity of the antenna models just over -40dBi with the counterpoise, and -35dBi if a good conventional earth is used.
?
So with a good conventional earth you may be able to go considerably smaller than 16m. which will help the antenna performance on higher frequencies where the K9AY cardioid pattern degrades. Quickly modelling a size reduction for near -40dBi on 1.8MHz indicates about 10.5m circumference. However, there is considerable difference in back-lobe pattern using the two earth systems, with a high null (around 45 degrees) using the conventional earth but less groundwave null.
?
In performance, the Aziloop is up to a few dB down in s/n on my beverages in their optimum directions on some days, as we might expect from their comparative RDFs, but this depends on direction and conditions of course. I can hardly rotate a 200m wire - !!? Also when the ionoshere favours higher angles then the Aziloop is just as good, again as you might expect from the comparitive patterns.
73
John
?
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Bill Riches
Christoph Ratzer