Antennas for HF reception

Question

I need to know a few things about using HF antennas with RTl SDR.

1. Whether it's a dipole or a monopole antenna, how do I choose the wavelength for the HF antenna? The range is from 3 MHz to 30 MHz. To make a dipole, it has to be 1/2 wavelength and 1/4 for the monopole. Choosing the lowest frequency (3 MHz) is the best thing to do, or should it be the focal frequency (14 MHz)?

2. For a monopole antenna, the ground plane driven elements should be connected to the ground (-ve) connector of the radio or it will work even without connecting?

3. What is the benefit of using a loading coil to a whip antenna? I made one with a loading coil of 50 meters long (please see the attached image "Monopole With Loading Coil.jpg").

4. I would also like to know how the other HF antennas for SW such as the Sony XHDATA AN-80 and Sangean ANT-60 work? What kind of antenna are these?

5. Some experts say that the gap between the two arms of a dipole should be between 1-2 cm but why is the gap between the rabbit ears whips more than that?

Answer 1

While resonate antenna are necessary for transmitting to prevent damage, receiving antennas will function, at a reduced sensitivity, even if the antenna is not a good match.

Dipoles have 2 halves, each a 1/4 wavelength for 1/2 wave length total. Verticals are usually 1/4 wave length. The other 1/4 wave length can be the ground itself, or a large conductive surface such as the metal top of a vehicle. For verticals mounted on top of a tower or pole, there are often multiple horizontal elements arranged radially around the base of vertical. If the ground is not very conductive, sometimes wires are arrange in a similar pattern across the gerund or buried just beneath it.

For simple dipoles and verticals I would try to pick one that matches the middle of frequency band you are trying to receive.

The ground elements of a vertical are not driven. We use the term driven most commonly with multi element antennas like Yagis. They are direction we refer to actual element connected to the transmitter / receiver as the driven element, the elements behind it as reflectors and the ones in front as directors.

put a coil at the bottom of an antenna adds inductance that make the antenna electrically look longer changing the resonance. This allows the use of shorter antennas. occasionally a capacitance is added to the top to do the same thing. This does reduce the sensitivity of the antenna.

the gap between the two legs of a dipole needs to be long enough to not from a capacitor. The exact distance can and does vary greatly.

Answer 2

Every radio antenna works in the same way. In receiving they catch a part of the radiowave. The caught part is directed along a wire or somehow more carefully designed transmission line into receiver's circuits. Often any separate transmission line doesn't even exist. The antenna is simply a wire or plate which is connected straight to the receiver circuitry. The electromagnetic field of the caught wave generates current to the circuit. It's filtered, amplified and finally converted to usable audio, video or data signal.

In transmitting operation AC current is fed to the antenna for example by connecting the antenna to an AC voltage source (radio transmitter is one) and the antenna emits an electromagnetic wave.The radiation can be remarkably strong if the antenna is properly engineered based on Maxwell's theory of electromagnetism. Or the antenna is made accidentally to be proper for its purpose. It can happen, but generally radio transmitters suffer badly (overvoltage peaks, heating) if the antenna is not properly designed. Radio receivers can often get an usable signal from an antenna which is not at all good to be connected to the output of a radio transmitter. That's because radio transmitters often are designed to use the parts at their limits and there's not much room for unexpectedly high currents and voltages.

In receiving one can easily lose say 99% of the theoretically available caught power (less than a microwatt) if the remaining 1% is enough for the receiver. A HF radio receiver can often detect signals from the other continent with a quite random wire antenna, like your linked Sony and Sangean types. If you try to send an answer with the same antenna you are lucky if your transmitter does the job. At least yo need an effective antenna tuning apparatus.

Designing an antenna is a demanding math challenge, far beyond the math skills of most of us. Radio amateur books present successful design results and there's as well numerous experimental results of successful tireless hobbyists. Use them. Half wave dipole and quarter wave whip are well known such results. Do not make lousy assumptions like "this antenna is said to be 50 % shorter than the theoretical quarter wave whip and it is still said working ok, because it has a lenghtening coil -so I need no whip at all if I have 2 coils in series!"

Rabbit ear whips in data networking routers and wireless mic receivers are separate antennas for separate radio receivers which are there to make possible to have more than one simultaneous radio paths. An alternative path increases substantially the probability of error free reception. In old portable FM radio receivers a pair of whips was a commercial gimmick to make the device look more attractive for a potential buyer. It was a materialized version of advertising terms "super", "hyper", "ultra", etc... In theory the 2 whips could be a half wave dipole, but the rest of the radio alone could be as good counterweight for one whip.