### Re: "Low Noise" Vertical Antennas for Receiving

Posted:

**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

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