"Low Noise" Vertical Antennas for Receiving

Useful information regarding antennas for SDR products.
SDRguy22
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Re: "Low Noise" Vertical Antennas for Receiving

Post by SDRguy22 » Sat Jul 20, 2019 11:04 pm

The purpose of this post is to present my views on a "Low Noise Vertical Antenna" conceived by Dallas Lankford.

An updated article describing this antenna can be found here: https://tinyurl.com/DallasLNV

I have been a SWL and ham for a long time and have always been interested in antennas. About ten years ago I came across Dr. Lankford's articles and found them quite interesting. He has been building, testing and publishing innovative receive antenna designs for MW and NDB DXing since the 80's. A Google search on this antenna and a visit to to the NDB Yahoo group is quite fruitful and many testimonials will be found about this antenna and his other designs. Some of the more well known and serious MW and NDB DXers have successfully used this antenna, his antenna amplifiers and other antenna designs. His work on dual antenna systems and phasers (used for interference cancelling) has been praised by technically competent reviewers and found to be very useful.

I just want to clarify a few things for the readers of this forum who might be interested in understanding how this electrically small antenna really works. The basic premise of this "Noise Reducing Vertical" is to lower the level of local noise pickup through the use of an impedance transformer at the antenna and a balanced feedline to the receiver. Reducing common mode noise on the transmission line is of prime concern. A short antenna like this has low radiation resistance, a high capacitive reactance, (thousands of ohms) and the ground connection, if radials are not employed, can be several hundred ohms. This results in a very high feedpoint impedance at the terminals of the antenna. In order to couple the voltage induced into the antenna, by the electromagnetic field around it, a high impedance termination at the antenna terminals of some kind needs to be employed.

One method is to use construct an active antenna with a high impedance amplifier like they do in automobiles, EMC antennas or HF active whips. Active antennas can be broadband and have decent gain but have several drawbacks. The first is that they require a power source and there can be noise introduced via the power feed. The second is that amplifiers are not perfectly linear and can result in unwanted spurious products due to intermodulation distortion (IMD). For MW DXers IMD can be a real problem when amplifiers are used due to the high field strengths that exist from local stations several kilometres away. Amplifier overload is also a concern on the congested MW band.

A second option is to use a high impedance transformer to get a better match to the high impedance of the short whip. The impedance at the transformer primary is equal to the secondary impedance times the square of the turns ratio. So a 10:1 turns ratio will have a 100:1 impedance ratio. The zip cord proposed by Lankford has a characteristic impedance of about 120 ohms at HF frequencies. So the antenna will see a 12K impedance. This input impedance is much lower than that of an active amplifier so there will be more signal loss particularly at lower frequencies. Another drawback to using an impedance transformer is that the secondary output voltage will be lower by the turns ratio and in this case will be 1/10 of the primary voltage. The net result is that this antenna will have considerable negative gain compared to an isotropic or dipole antenna. Lankford states -15 dB in his article and that is similar to some Beverage antennas which are also popular for under 2 MHz. reception. When compared to a Wellbrook Loop, which is a popular MW and HF antenna the gain is 12 dB lower at 1 MHz.

For receive applications the goal is to have sufficient antenna gain so that the atmospheric or local noise is greater than the noise floor of the receiver. The second objective is to minimize local noise pickup as much as possible. This antenna has sufficient antenna gain for MW reception (with high field strengths) and LF reception (high atmospheric noise) given today's sensitive receivers and meets these objectives. Lankford also designed and published a companion 11 dB. low noise figure amplifier with excellent IMD performance for those desiring additional gain. It is placed at the receiver end.

Those interested in reading further on this subject will find a collection of Dallas Lankford's work at this link.

https://tinyurl.com/dallas-collection

Thanks to Ian1951 for bringing this antenna to our attention. Mike2459's comments on this antenna were also interesting as was the article he posted by Steven Best titled "Optimizing the Receiving Properties of Electrically Small HF Antennas".


Bill

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Mike2459
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Re: "Low Noise" Vertical Antennas for Receiving

Post by Mike2459 » Sat Jul 20, 2019 11:59 pm

Bill

I'm glad to see someone else give Lankford the respect that he deserves. I found his articles on flag antennas arrays particularly interesting.

I'm responding to your request in the earlier DCF77 thread: "From what I understand you have designed an active antenna and I hope you can post a little more about it (some pictures and/or a schematic)."

I didn't design a miniwhip, I've built two, one based on PA0RDT's design the second from a design used by PA3WFM at the University of Twente. I used a vintage Motorola 2N5160 UHF transistor in place of the BFQ149 and a SMD type J310 as the input FET. (I couldn't find any BFQ149's.)
The performance of both antennas are about the same. The purpose of building the 2nd was to achieve better IMD performance, it does pretty well in that respect as long as the supply voltage can be maintained at 12 volts at the antenna.

Here is the link to the second one: http://www.pa3fwm.nl/projects/miniwhip/

Mike

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M0RON
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Re: "Low Noise" Vertical Antennas for Receiving

Post by M0RON » Sun Jul 21, 2019 7:10 am

Thank you Bill for your post and the links it contained.
I'll spend some time this week looking through it all and maybe think about building one to compare to my PA0RDT which works very well at my location.
Andy

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ON5HB
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Re: "Low Noise" Vertical Antennas for Receiving

Post by ON5HB » Sun Jul 21, 2019 4:01 pm

Hi Bill,

He makes a very interesting point, but sadly a wrong remark:
While studying active whip intercepts some time ago I discovered, much to my
amazement, that long coax (50 feet) lead often degrades 2 nd order intercepts of active
whip antennas by 20 dB or more and degrades 3 rd order intercepts of active whip
antennas by up to 10 dB, depending on the type of active whip antenna. I have not
studied the cases of longer coax lead in, or long coax lead in used with active dipoles,
or long coax lead in used with (passive) noise reducing antennas. For active whips
long (50 feet) twin lead lead in does not change 2 nd or 3 rd order intercepts. Also, I have not studied the cases for
longer twin lead lead in with active whips, or for twin lead lead in used with active dipoles or (passive) noise reducing
antennas. I rather expect that coax lead in will be a loser with respect to intercepts in all of those cases, while twin
lead lead in will be a winner with respect to intercepts in those cases. Of course, if your antenna is not in a high RF
environment, then it probably won't matter if you use coax lead in. On the other hand, more recently I have found that
coax lead in can cause substantial man made noise in active whip antennas compared to twin lead lead in. It appears
that the coax induced noise is via common
mode,
but unfortunately the noise is virtually
impossible to eliminate completely throughout
the MW band even with multiple common
mode chokes. I am beginning to understand
more clearly why active whip antennas have
such bad reputations wrt man made noise. All
except mine use coax lead in as well as DC
power feed.
I marked it, that is exactly the problem with coax on HF.
But it's also very easy to eliminate and once eliminated the mini-whip has little man-made-noise but the reception is poor compared to a dipole.

My problem is that he's rather anti-coax instead of doing it right.
Coax and twin-lead a basically the same if the 3th current on coax is removed, sadly all too many people ignore this and simply dismiss coax as being a good conductor for VLF/HF.
Coax with a proper common-mode-solution has no issue with high-RF in the neighbourhood, none at all.

Then he goes on:
If implemented correctly, active whip antennas can be as immune to man made noise as loops or any other antennas,
despite claims to the contrary.
People who have made those contrary claims probably did not use common mode
3chokes, or low noise AC/DC power supplies, or find a low noise location for the whip; see, for example, here for John
Plimmer's interesting man made noise experiences with a loop and an active whip.
I dare to say, ANY antenna with coax can be made immune to man made noise when the 3th current is removed.

His approach isn't much different from a mini-whip but he uses twin-line so he hasn't have to deal with common-mode-problems, that is why his design has less issues as most mini-whip users do not tackle common-mode-currents and as such the mini-whip performs badly.
BTW, he also writes this :lol:
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SDRguy22
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Re: "Low Noise" Vertical Antennas for Receiving

Post by SDRguy22 » Thu Jul 25, 2019 8:11 pm

ON5HB wrote:
Sun Jul 21, 2019 4:01 pm
Hi Bill,

I dare to say, ANY antenna with coax can be made immune to man made noise when the 3th current is removed.

His approach isn't much different from a mini-whip but he uses twin-line so he hasn't have to deal with common-mode-problems, that is why his design has less issues as most mini-whip users do not tackle common-mode-currents and as such the mini-whip performs badly.
I agree with you Bas. Common mode current on the outside shield of a coaxial cable can be a problem but can be remedied with ferrites, isolation transformers and proper grounding. I think the other reason for not using coax in this design is to keep the impedance high on the primary of the transformer.

Bill
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ON5HB
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Re: "Low Noise" Vertical Antennas for Receiving

Post by ON5HB » Fri Jul 26, 2019 11:31 am

SDRguy22 wrote:
Thu Jul 25, 2019 8:11 pm
I agree with you Bas. Common mode current on the outside shield of a coaxial cable can be a problem but can be remedied with ferrites, isolation transformers and proper grounding. I think the other reason for not using coax in this design is to keep the impedance high on the primary of the transformer.
Hi Bill,

As for impedance, I look at it in a total and not just the "typical" impedance of a transmission-line as such.
I mean, just look at the way they measure typical impedance of cable. It's old and outdated.

A very good read on this is Reflections by M. Walter Maxwell, W2DU/W8HKK.
I consider it the bible on these matters. It's also very funny as most books for Ham's are as dry a old bread :lol:
M. Walter Maxwell wrote: Copernicus persuaded the multitudes that the universe did not revolve around the earth.
This situation originated with the introduction of coaxial transmission lines for amateur use around the time we got back on the air after World War II, and has gained momentum since SWR indicators appeared on the scene and since the loading capacitor of the pi-net tank replaced the swinging link as an output-coupling control.
We are in this state because so much misleading information has been, and is still being published concerning behavior of antennas which are not self-resonant, feed-line performance in the presence of reflections when mismatched to the antenna, and especially the meaning and interpretation of the VSWR data.
Articles containing explicitly erroneous information and distorted concepts find their way into print, become gospel, and continue to be propagated with chain-letter effectiveness.
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ernestyn
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Re: "Low Noise" Vertical Antennas for Receiving

Post by ernestyn » Fri Jul 26, 2019 1:12 pm

Hi ON5NB I have seen your posts and wonder if you can help me with the special data of the capacitor smallest antenna described in this collection. I am also interested in no reflection measurement of antennas, as you are exposing, as I believe that for new small capacitor antennas very new special measurement needed because with no amplifier measured reflection of is high with normal methods.
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ON5HB
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Re: "Low Noise" Vertical Antennas for Receiving

Post by ON5HB » Fri Jul 26, 2019 4:28 pm

Small capacitive antennas are called Mini-Whip they work well at very low frequencies, it all depends on the length of the whip.
As for reflections, I do not measure it, I have a tuner for transmitting that solves mis-matches.
For reception it doesn't matter much if there is a mismatch, the RSP receivers are more then sensitive enough to resolve the small losses.

What bands are you interested in? There is no single optimal multi-band-antenna that is also small.

In these cases: Size does matter :lol:
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ernestyn
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Re: "Low Noise" Vertical Antennas for Receiving

Post by ernestyn » Sat Aug 03, 2019 1:08 pm

Dear ON5HB, thank you for your reply, but I do not undesrtand. I am researching wide band antennas and the description for the miniwhip and all discussions I have read in the forum say we have here a wideband and also low noise antenna, like I am also researching, based on the work on capacitive probe by Dr. Y. Zehforoosh. Why you now say it is not band wide? Please explain, because it is very interesting about the change in size you mention as per theory.


Also it is true your discussion about SWr because a projected omopolar capacitive antenna is like an infinite impedance source and swr is useless.
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ernestyn
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Re: "Low Noise" Vertical Antennas for Receiving

Post by ernestyn » Sat Aug 03, 2019 4:49 pm

Dear SDRguy22 I read your interesting post on low noise vertical antennas and I have studied the important work of Dr. Langford. I am interested in lowest noise widebandest antennas and I am working with the capacitor theory for the wideband. Now the schematic of Dr. Langford cannot be capacitor as I see ground reference as shown in the schematic I copy below.
Scematic widebandest.png
Scematic widebandest.png (135.57 KiB) Viewed 4711 times
It says:
works MW, LW, and SW - Works Well at most Short Wave Frequencies too
So it works all over, but current must flow in transformer because of ground connection, but then worse noise come in and also no capacitor, so how can be bandwidest? You have studied & used this antenna & please help explain me.
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