Compact highly directional medium wave antenna designs?

[Flyboy]

I know that there are highly directional shortwave/HF and VHF antenna designs that can be made fairly compact, but is it possible to do the same for medium wave, i.e. below 3MHz? I can’t seem to find anything helpful online about any theory that would point to a shape, let alone an actual example.

Is this due to the wavelengths involved?

 

[berkeman]

RX only, or RX/TX?

 

[Flyboy]

RX only. The intent is to use it for direction finding using 1940s technology.

 

[berkeman]

The intent is to use it for direction finding using 1940s technology.

How compact and how accurate? I found some new DF instruments that can operate down that low in frequency, but they are still 10 meters or so in terms of antenna separation, and use pretty sophisticated new technology:

https://www.morcom.com/direction-finding-systems

WD-6300 HF Direction Finding System

This system is designed to provide precise direction finding (DF) for HF signals. It has a frequency range of 500KHz to 40 MHz. It employs a sophisticated phase-coherent multi-channel interferometer-based method, combined with statistical signal processing. This delivers a cost-effective, robust, and precise HF DF solution. This system is designed for the for government, military, and law enforcement applications that require geo-locate HF emitters.

The system is small, lightweight, ruggedized, making it suitable for deployment in stationary and portable situations. Each WD-6300 system consists of three core parts: antenna array, three phase-coherent receiver channels, and associated processing computer.

The WD-6300 is a true none-commutated system that provides excellent performance against short duration signals. The sensing antennas utilized are specially adapted AX-81SM active monopoles. The antenna array size can simply be adjusted to match the system's desired frequency range. The antennas are positioned in a “L”-shape configuration, with each leg typically 5 meters in length for optimum coverage from 3 MHz to 30 MHz, and typically 10 meters for best coverage between 1 MHz to 15 MHz.

In addition to precise azimuth measurements, the WD- 6300 system can also measure an incoming signal's elevation. Providing the height of the reflecting ionospheric layer allows for an estimation of the target’s location without resorting to a standard multi-sensor approach for the necessary triangulation.

The entire antenna array can be assembled in less than thirty minutes by two people, while the receivers and the processing computer system can be operated either stationary or installed inside a suitable vehicle.

 

[Baluncore]

RX only. The intent is to use it for direction finding using 1940s technology.

There are many possible solutions. The biggest problem is that HF signals wander ±2° in azimuth and ±30° in elevation due to ionospheric changes. You will need to trade gain for directivity, because HF DF accuracy comes from the narrow deep null, not the insensitive main lobe. Almost all HF DF techniques were operating during WW2. Many during WW1. Only interferometry using fields of small loops have emerged more recently.

A single, rotatable, tuned loop, will have a deep null.

Two rotatable loop antennas, mounted together with 5° between their axes, switch between them at 1 kHz, detect the 1 kHz in the received signal, and null that. The inverse system was the Lorenz beam landing system and Knickebein.
https://en.wikipedia.org/wiki/Battle_of_the_Beams
https://en.wikipedia.org/wiki/Battle_of_the_Beams#Knickebein

Two travelling wave wires, in a fan towards the target. Install series capacitors in the antenna wire, to give super-luminal velocity factor over the ground plane. That can have such a fine beam that you cannot find the target, back off a bit on the vf.

Crossed Bellini-Tosi loops, a two channel receiver, with one LO, feeding the x-y plates of an oscilloscope. Often used for broadband lightning direction detection.
https://en.wikipedia.org/wiki/Bellini–Tosi_direction_finder

HFDF, Huff-Duff, used to find submarines in the Atlantic.
https://en.wikipedia.org/wiki/High-frequency_direction_finding
https://en.wikipedia.org/wiki/High-frequency_direction_finding#Description

 

[Flyboy]

How compact and how accurate? I found some new DF instruments that can operate down that low in frequency, but they are still 10 meters or so in terms of antenna separation, and use pretty sophisticated new technology:

https://www.morcom.com/direction-finding-systems

Something that you could fit two of to a fighter-sized airframe with 1940s technology. I know they could do that easily enough with the electronics, but they required a navigator to make sense of the data.

Accuracy… as long as it’s relatively close, i.e. able to use two to triangulate within a mile or two is fine.

There are many possible solutions. The biggest problem is that HF signals wander ±2° in azimuth and ±30° in elevation due to ionospheric changes. You will need to trade gain for directivity, because HF DF accuracy comes from the narrow deep null, not the insensitive main lobe. Almost all HF DF techniques were operating during WW2. Many during WW1. Only interferometry using fields of small loops have emerged more recently.

A single, rotatable, tuned loop, will have a deep null.

Two rotatable loop antennas, mounted together with 5° between their axes, switch between them at 1 kHz, detect the 1 kHz in the received signal, and null that. The inverse system was the Lorenz beam landing system and Knickebein.
https://en.wikipedia.org/wiki/Battle_of_the_Beams
https://en.wikipedia.org/wiki/Battle_of_the_Beams#Knickebein

Two travelling wave wires, in a fan towards the target. Install series capacitors in the antenna wire, to give super-luminal velocity factor over the ground plane. That can have such a fine beam that you cannot find the target, back off a bit on the vf.

Crossed Bellini-Tosi loops, a two channel receiver, with one LO, feeding the x-y plates of an oscilloscope. Often used for broadband lightning direction detection.
https://en.wikipedia.org/wiki/Bellini–Tosi_direction_finder

HFDF, Huff-Duff, used to find submarines in the Atlantic.
https://en.wikipedia.org/wiki/High-frequency_direction_finding
https://en.wikipedia.org/wiki/High-frequency_direction_finding#Description

Elevation is a non-issue, but azimuth accuracy is important.

Basic premise is trying to come up with a plausible passive terminal guidance system for an early nuclear-tipped cruise missile for an alternate timeline. The concept is to use two direction finders to get a read on two separate commercial radio stations to figure out the target point after cruising on autopilot to get close. This is only supposed to be an interim measure, as widespread rollout of the VOR navigation network is still a few years down the road.

Basic concept schematic:

 

[berkeman]

"Close only counts in horseshoes, hand grenades, and, well, ..."

nuclear-tipped cruise missiles

 

[Flyboy]

Correct.

It’s not meant to be a pinpoint system, but enough to make sure you’re in the right area. A purely internal system like inertial navigation would not be practical at the time, and the computing power to do triangulation along the flight path to correct for navigation errors would be prohibitively expensive and bulky to use on a throwaway system. I figured that using the angle between two known, fixed emitters in the target area would be a fairly viable option.

Downsides are that if your enemy catches wind of your guidance concept, they will shut down the radio stations if they know an attack is imminent, denying you the ability to guide off of them, and the fact that you basically get one shot per city. After all, the nuke is likely to disable or destroy the radio towers in the area.

 

[berkeman]

So it sounds like this is for a SciFi story or similar? It's probably a specialized enough question that we can keep this thread in the EE forum for now.

 

[Flyboy]

So it sounds like this is for a SciFi story or similar? It's probably a specialized enough question that we can keep this thread in the EE forum for now.

Alternate timeline story, but yes.

The technical aspects are exactly why I placed it here instead of in the Scifi world building forum.

 

[Baluncore]

I figured that using the angle between two known, fixed emitters in the target area would be a fairly viable option.

That will map to a hyperbola, not a point on the ground.

Two points need a reference direction. Is your reference from an inertial system or a gyrocompass that was spun up before launch ?

No compass is needed with a three point resection, only two of the angles between the three points need to be measured.
https://en.wikipedia.org/wiki/Position_resection_and_intersection

MW transmitters transmit horizontally, towards the horizon, there is no signal directly above, so the system may be blind.

V or H polarisation of the received MF wave, will require a crossed dipole antenna for the receiver, maybe setup for circular polarisation, so orientation is unimportant and only one receiver is needed.

 

[Flyboy]

That will map to a hyperbola, not a point on the ground.

I’m not sure I understand. If I know that the bearing from the target to one station is, as in the example diagram, 130° magnetic, and the second one is at 210°, that produces an angular difference of 80°, correct? If you know which one is which based on the frequency and know you’re coming in from one side of the target, wouldn’t that leave only one possible point where the bearings match up? I’m operating on the assumption (and I’m fully aware of the danger that represents ) that I can use the axis of the aircraft’s travel as a third angle. So, I can tel that Station A is ahead of me, and Station B is on my right, and I can measure the difference between the two DF seekers to tell when it’s the right place.

Two points need a reference direction. Is your reference from an inertial system or a gyrocompass that was spun up before launch ?

No compass is needed with a three point resection, only two of the angles between the three points need to be measured.
https://en.wikipedia.org/wiki/Position_resection_and_intersection

I was thinking of using a fluxgate compass paired with a gyrocompass. It’s available at the time, if a bit new, but it simplifies gathering direction data. A manually input magnetic declination adjustment would be part of setting up the missile prior to launch.

I’m trying to make it as simple and compact as practical. A bit brute force, as it is intended to be used by the Soviets.

MW transmitters transmit horizontally, towards the horizon, there is no signal directly above, so the system may be blind.

V or H polarisation of the received MF wave, will require a crossed dipole antenna for the receiver, maybe setup for circular polarisation, so orientation is unimportant and only one receiver is needed.

I don’t expect to be using direct overflight of a station as a navigation tool, but it certainly could be useful in later designs to provide a known point for INS realignment.

As for polarization… whatever gives the best angular resolution for US AM radio broadcasts.