Re: Lw/ Mv and 160m.. which rx antenna?

Hi Simon,

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Unfortunately, I think you are stuck with needing something relatively large.

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I previously mentioned 8 phased small loops, this was to provide sufficient S/N and RDF, so as to be substantially better than your other choices.

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The most useful slide from the YouTube presentation, is probably the one showing the performance of various receive antennas relative to each other.

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I have placed a copy in the group photos section.

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/g/loopantennas/photo/267767/3899305?p

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Your Tee and magnetic loop, probably have an RDF of around 5 to 6dB.

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The Waller Flag sits at around 11.5dB, which is similar to a 1000ft Beverage, or an active 8 Square, both of which would require a lot of space to deploy.

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Regards,

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Martin

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On Mon, Mar 24, 2025 at 04:47 PM, Simon wrote:

Show quoted text

Rather large,

Re: Lw/ Mv and 160m.. which rx antenna?

#21238

Rather large, plus a fair few negative reviews..ie gone back to a k9.

I do have  large mag loop at end of garden. ( total circumference 20m of no joins copper tubing.) My ex London qro loop..will try that outpointing towards Vk.. nothing to lose.
Simon g0zen

Re: Lw/ Mv and 160m.. which rx antenna?

#21237

An antenna no one knows about, anymore, it has disappeared from public view and has no supporters, but has allowed stations to work DX they could not have worked without it.

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I speak, of course, of the Waller Flag.

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Here is a presentation from those actually using it:

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Strict attention to detail regarding coax and noise intrusion when constructing the preamps is key, as explained in the presentation.

Re: Lw/ Mv and 160m.. which rx antenna?

#21235

Hi Martin

I tend to agree with you. Even with 8 phased loops they will be “below” house gnd level in rhe direction interested in.
Would only be able to mount 1 on house roof level. 

Next month am improving the T, the one  25m top wire is being replaced with 3. ( so will look like a mw tx antenna as used by broadcast stations.) This will allow less L on loading coil. “ every bit helps.” Last night worked a good few na and as ru stations on 160m using it..shouldnt complain really.

So unless someone has a plan for a small miracle rx antenna I guess will leave as is..( rx antennas here. Which are 3 crossed parallel loops, each loop made of 4 1m square loops, feeding lz1aq 5551 style preamps and a lz1aq designed 4 way directional delay line box. Thus giving me 2 phased inline loops in 4 directions at any one time. 

A flag,  k9ay, short vertical  etc wont be any better due to where it would be mounted ( down the garden, near my tx antennas and they all appear to need to work against gnd or elevated radials, not practical on the roof! 

Thanks for reading Simon g0zen

Re: Lw/ Mv and 160m.. which rx antenna?

#21234

Hi Simon,

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I think it is unlikely that your small loops, in such a low noise environment, will have sufficient "sensitivity" to be constrained buy your local nose floor, which is probably why your Tee seems to perform better. Phasing more small loops my give you the required directively within the space available to you, but I suspect that 8 or more may be required in order to make an appreciable difference.

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The long (ish) wavelength of antennas on 160m, and their interaction with surrounding objects, tends to be relatively stable and predicable, as is propagation.

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I suspect your observations may be being influenced by other factors, such as the underlying geology of the area surrounding your location, and other stuff much further away from the actual antenna.

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E-Probes are problematic, as their unbalanced, high impedance feed, tends to make them a "noise magnet", and you would still have to phase them to obtain the required directively.

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Just my passing thoughts.

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Regards,

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Martin

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On Sun, Mar 23, 2025 at 02:15 AM, Simon wrote:

Show quoted text

Any thoughts???

Lw/ Mv and 160m.. which rx antenna?

#21233

Hi

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been playing around with ft8 on 160m.. on psk reporter heard in vk etc.. ( when band open.)

tx and rx antenna is a Marconi T. However rx in said directions from here is poor. ( see below.)?

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Live on dartmoor national park ( just,) nice low noise floor. Nice and high (asl)?

I have crossed parallel loops, delay lines and Lz1aq (5551’s) preamps for rx if I wish.

Thing is on 160 the Maroni T beats them..?

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Now I need to state, my garden is on a BIG slope ( Dartmoor) my rx array is 10m below gnd level of house basement.(20m below roof line.) ?My garden slopes to west..

Excellent results from south to north, due to clear take off, but though north/ east to south not so ?good due to antennas below gnd level of house.

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so i ?need to improve rx on 160 towards VK etc.?

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Thinking another singular loop on roof ( have pole already erected. ?Or an short vertical eprobe.?
Have ?never used an eprobe, so no idea of performance..

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Any thoughts???

please see my qrz page for pic of garden to get idea.bottom of garden is to west of house. Bottom of garden ( gnd) is 20m below roof line.

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Simon g0zen?

SAL - 20 for sale

#21232

I have a SAL-20 antenna for sale.? Contact me off list for details.

73l

Bill, WA2DVU

Cape May. NJ

609 425 8651

bill.riches@...

Re: File /MODIFIED AMRAD PREAMP - 20 MARCH 2025.docx uploaded #file-notice

#21231

@Dave, Martin

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i am on a weekend trip. Will be back soon and try to answer your questions.

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regards, Fred

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Re: File /MODIFIED AMRAD PREAMP - 20 MARCH 2025.docx uploaded #file-notice

#21230

I couldn't agree more, Fred.? Audio, yes. ... RF, likely not.? But what about below 100 kHz where most of my application lands?? I'm primarily interested in "DC" through 100 or so kHz.

Dave - W?LEV

Show quoted text

On Fri, Mar 21, 2025 at 7:53?AM Fred M via <dl4zao=[email protected]> wrote:

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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regards

Fred

--

Dave - W?LEV

Re: Modified AMRAD

#21229

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:

Show quoted text

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)

Re: Modified AMRAD

#21228
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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/g/loopantennas/files/111AAATemporaryFiles/DL4ZAO_Hi-Z_active_Whip_amplifier.pdf

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regards

Fred

Re: Modified AMRAD

#21227

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

Re: Modified AMRAD

#21226

开云体育

?? 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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Show quoted text

----- Original Message -----

From: Martin - Southwest UK via groups.io <martin_ehrenfried@...>

Reply-To: <[email protected]>

To: <[email protected]>

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.

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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.

Re: Modified AMRAD

#21225

开云体育

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

Show quoted text

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

?

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.

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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,

?

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.

Re: Modified AMRAD

#21224

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:

Show quoted text

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

Re: Modified AMRAD

#21223

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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Re: Modified AMRAD

#21222
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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