Crystal Radio
Radio Frequency Bands
Antenna Length Calculations
Designing a Dipole Antenna
Adding a Strong Ground Plane
The Importance of Ferrite Beads/Torroids for Radios (HAM or CB)
Filtering EMI by Using a Faraday Cage
Motorcycle Battery as Power Source
Morse (CW) Learning Method
The 30/70 Rule
Sound Card Disconnecting during Transmit
Antenna Voltage Blocking Capacitor

Crystal Radio

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Radio Frequency Bands

The amplitude modulated (AM) radio carrier frequencies are in the range $535-1605kHz$ and used for maritime communication and aircraft navigation. Carrier frequencies $540$ to $1600kHz$ are assigned in $10kHz$ intervals.
The shortwave band or RF range between $1605kHz$ and $54Mhz$ .
- $1605kHz-30Mhz$ amateur radio, government, international shortwave broadcast, fixed and mobile communications.
- $30-50Mhz$ government and non-government, fixed and mobile such as police, fire, forestry, highway and railroad.
- $50-54Mhz$ amateur radio.
- Nuclear magnetic resonance (NMR) and magnetic resonance imaging (MRI) fall in the $40-50MHz$ range and it is the proton resonance frequency range.
TV channels 2-4 in the range $54-72Mhz$ .
$72-76Mhz$ reserved for government and non-government with a standard aeronautical beacon at $75Mhz$ .
TV channels 5 and 6 between $76-88Mhz$
FM radio between $88-108Mhz$ .
$108-122MHz$ for aeronautical navigation including localizers, radio ranging and airport control.
$122-174Mhz$ for government and non-government signals including amateur broadcasts.
TV channels 7-13 span between $174-216Mhz$ .
$216-470Mhz$ fixed and mobile communications including aeronautical navigation and citizens radio.
$470-890Mhz$ UHF television channels 14 to 84.
- Cellular phones $824.040-848.970Mhz$ overlap UHF.
- $390-1550Mhz$ L-Band, satellite communication (including GPS).
$890-3000Mhz$ various aeronautical and amateur uses, studio-transmitter relays.
- Radar bands between $1300-1600Mhz$ .
- $390-1550Mhz$ L-Band, satellite communication (including GPS).
- $2450Mhz$ microwave ovens.
- $1.6-30GHz$ 3K background radiation ( $187-10mm$ ).

Antenna Length Calculations

The optimal length $l$ in meters for an antenna designed to receive signals with a frequency of $\lambda Mhz$ is given by the formula:

$\begin{eqnarray*} l &=& \frac{300}{\lambda} \end{eqnarray*}$

where $300$ is the approximate speed of light in vacuum simplified to match the frequency in megahertz.

For instance, for a $2.4GHz$ signal, the optimal length will be:

$\begin{eqnarray*} l &=& \frac{300}{2400} \\ &=&0.125m \\ &=&125mm \end{eqnarray*}$

However, since the wavelength can be represented as a sine function, any multiple of $l$ will still be able to pick up the signal efficiently.

Designing a Dipole Antenna

Given a frequency range divided into channels (such as CB radio channels) an antenna can be build that matches the mid-point of the frequency range corresponding to the middle channel (ie: for EU, 40 channels, channel 20).

Let the lower frequency of the range be $\lambda_{l}$ and the high frequency of the range be $\lambda_{h}$ such that the mid-point frequency of the range is $\frac{\lambda_{l} + \lambda_{h}}{2}$ . Substituting into the length of the antenna formula from the previous section, we obtain:

$\begin{eqnarray*} l &=& \frac{300}{\frac{\lambda_{l} + \lambda_{h}}{2}} \end{eqnarray*}$

which can then be further reduced to the following formula:

$\begin{eqnarray*} l &=& \frac{600}{\lambda_{l} + \lambda_{h}} \end{eqnarray*}$

A dipole antenna consists in two diametrically opposed radiating elements of equal length where the total in-line length of both elements is the length or a fraction of the wavelength calculated using the previous formula. For instance, the following schematic represents a full-wave dipole antenna with both elements carrying a half wave.

Substituting for a citizen band radio operating within the EU:

$\begin{eqnarray*} l &=& \frac{600}{26.965 + 27.405} \\ &=& \frac{600}{54.37} \\ &\approx& 11.035m \end{eqnarray*}$

Dividing the full length by $2$ in order to obtain the total size of each individual element on the dipole antenna:

$\begin{eqnarray*} \frac{l}{2} &\approx& \frac{11.035m}{2} \\ &\approx& 5.51m \end{eqnarray*}$

In case $5.51m$ for each element is not a manageable size, a quarter-wave can be achieved by dividing the result further by two $\frac{l}{4} \approx \frac{5.51m}{2} \approx 2.75m$ thereby obtaining a shorter and more manageable length.

Adding a Strong Ground Plane

SWR can sometimes be reduced by adding proper grounding to the antenna. A copper ring is created that can be attached around an SO239 female jack and a lead drawn to be fastened to the ground.

In this instance, power is provided to a CB radio via a PSU such that the ground is attached to the chassis which, in turn, is connected to the ground pin of the kettle power cable.

The Importance of Ferrite Beads/Torroids for Radios (HAM or CB)

Operating a radio during a transmission has the implicit effect of turning all the cables attached to the radio into antennas. In turn, the cables then tend to generate RF interference for all neighboring devices. The interference scales with the amount of power used during transmission.

For instance, when operating a radio in close proximity to a computer rig, it may be that odd behaviours might be exhibited by various neighboring equipment, such as monitors or even Bluetooth devices, during transmission.

In order to prevent such interference there exist various solutions yet all the solutions hinge on the concept of a low-pass filter. That is, a low-pass filter has the effect of only letting an useful range of frequencies through, whilst attenuating frequencies over the upper limit of the low-pass filter specification. For example, operating a citizen or chicken band (CB) radio (11 meters), all channels within the frequency range are to be typically found between $26MHz$ and $28MHz$ such that any other frequency above (or below) is not only useless to the CB radio operator and other receivers, but could also be considered pollution and might interfere with neighboring devices.

There are two solutions that can help reduce the interference of neighboring devices:

an advanced low-pass filter tailored specifically to the frequency range that the radio is designed to operate on (this might be a high cost solution),

ferrite beads

In theory, ferrite beads should be calculated to match the required frequency range of the radio, in order to only attenuate unneeded interference but in practice, most ferrite beads on the market would do since their cutoff point lies well after CB as well as HAM radio bands. Similarly, for the equipment being interfered with, modern devices are typically low-power devices (requiring a just a few Volts to operate) such that a terse attenuation is not even required.

The most nocive devices that typically produce interference are trivially devices that rely on heavy oscillations - for instance, a car alternator or a ceiling fan, or any other mechanical device (not capacitors within digital circuits, that typically operate on just a few Volts).

There are several types of ferrite beads that can be used to hamper high frequencies and thereby interference. Clip-on ferrite beads do not require the ending of the cable to be cut off such that they might be the best solution whilst torroids can be used in cases where the cables can be passed though. When wrapping a cable, attempt to pass the cable multiple times through the ferrite bead instead of just once in order to create a coil. The cables that should be wrapped are the cables that connect the radio (in any way) to any other equipment. Similarly, the antenna itself should not be wrapped, in particular if the radio is a VHF radio that operates on frequencies that the ferrite beads would hamper.

Ferrite beads can usually be salvaged from various equipment cables, torroids can be found within computer power supplies (due to low-EMI regulations) but can also be bought.

Filtering EMI by Using a Faraday Cage

Similar to the previous section about reducing EMI interference using ferrite beads, it might happen that a high powered emission might also influence other equipment.

A Faraday cage is typically constructed out of metal and acts as a low-pass filter where frequencies above a given frequency are radiated into the metal frame and do not surpass the bounds of the cage. Even if the original experiment of Michael Faraday involved a cage, an ideal Faraday "cage" could possibly and ideally consists in a solid metal enclosure. However, sometimes it is preferable to use a "cage" with holes mainly for decreasing the costs. Nevertheless, there is a relation between the size of the holes in a Faraday cage and the frequencies being transmitted from inside the cage.

In order to calculate the build of the cage, a quick mnemonic is that a Faraday cage must not have holes that are larger than $\frac{1}{10}$ of the wavelength being emitted.

In other words, the maximal dimension of the holes in a cage $d$ , measured in meters, is given by the formula:

$\begin{eqnarray*} d_{F} &=& 1/10 * \gamma \end{eqnarray*}$

where:

$\gamma$ is the wavelength

To calculate the dimension of the holes of a Faraday cage, it is known that the frequency is defined by:

$\begin{eqnarray*} f &=& c/\gamma \end{eqnarray*}$

where:

$c$ is the speed of light,
$\gamma$ is the wavelength in meters

and solving for the wavelength $\gamma$ :

$\begin{eqnarray*} \gamma &=& c/f \end{eqnarray*}$

the wavelength ( $\gamma$ ) is obtained relative to the speed of light $c$ and the frequency of the transmission $f$ .

Typically, for ham radio operators, the wavelength is known. For instance, CB radio frequencies fall within the $11m$ band. For instance, using the formula:

$\begin{eqnarray*} \gamma &=& c/f \end{eqnarray*}$

and substituting the speed of light in vacuum, $c \approx 299 792 458 \frac{m}{s}$ and, let's say, the mid point frequency of the European CB band, that is, channel 20, $f = 27205000Hz$ , the equation solves as:

$\begin{eqnarray*} \gamma &\approx& 299792458/27205000 \\ &\approx& 11.0197 \end{eqnarray*}$

Using the mnemonic to compute the maximal hole size that a Faraday cage may have in order to stop all frequencies above the CB $11m$ band, substituting all known values and solving for the dimension of the maximal hole size in a Faraday cage $d_{F}$ :

$\begin{eqnarray*} d_{F} &\approx& 11/10 \\ &\approx& 1.1 \end{eqnarray*}$

it turns out that a cage that would not allow emissions higher than the $11m$ band should not have holes that are larger than $1.1$ meters.

For all practicality, for HAM radios, very fine holes are not required and the hole size is allowed to be large, up to the UHF bands ( $1m$ and more) where the formula should be applied to make sure.

Here is a Faraday cage built for a Yaesu FT-891 radio, with the intent of containing any residual emissions within the cage and ensuring that the signal will pass through the antenna instead of being radiated in all directions from the radio.

For shoppers, the cage was cheap to find and it is no more than a cheap cage for small pets with the only condition that the cage should be built out of metal and not plastic.

As can be observed, the power source is also kept within the cage - and why not, given that switching power supplies do generate residual emissions that could just as well be contained by the same cage. Intuitively, in case an USB hub would have to be used, compared to the power supply, the USB hub should be placed outside of the cage due to the EMI generated inside the cage being able to influence the circuitry within the USB hub.

Motorcycle Battery as Power Source

One very cool solution for both portable and fixed station HAM radios is to purchase a motorcycle battery. These batteries are typically cheap acid based batteries that will last forever in case the only device that drains the battery is the radio. For instance, terminal rails can be attached to the battery in order to offer an easy way to connect devices to the battery.

Similarly, in order to charge the battery, a typical car battery charger can be used to recharge and maintain the battery. Doing so would reduce the noise level to nothing as well as decoupling the radio from other nearby circuits such as a landline.

Morse (CW) Learning Method

Most experts tend to claim that morse is best learned audibly rather than visually. For instance, even if morse is learned to the point where an operator can broadcast the letters by memorizing the dits and dahs sequences, it is not a guarantee that the same operator can do the reverse and transform the sound patterns back into letters.

That being said, perhaps the following method could yield better results for operators wishing to learn morse code. The method consists in loading up the following clips in a player and then shuffling the playlist such that all letters will be played randomly. The clips consist in the spoken letter, followed by its corresponding dit and dah sequence. In doing so, an operator willing to learn morse, could just listen to the same playlist ad nauseum whilst performing other tasks until, hopefully, the letters and corresponding morse sequence sounds are memorized.

To use the files, download each clip and add them to any player that supports shuffling (or perhaps go through the list alphabetically at first) and play them back.

The original clip is made by greedonever and the letters have been cut up separately in order to be able to listen to the letters individually.

The following list is the reverse of the former where the morse sounds are heard first, followed by the corresponding letter. Ideally, this should stop bias where a listener would just agree with the spoken letter even if the sound is not yet heard.

The 30/70 Rule

As a general trope, the 30/70 rule highlights the fact that only $30\%$ of a good setup consists in the actual HAM radio hardware and that $70\%$ is left over for the antenna. In other words, even if one were to purchase extremely expensive hardware, a solid antenna tuned for the correct band is at least $40\%$ more important. Quality HAM radios typically have very many extra features bolted on that are not really needed and barely used. Both sending and receiving, as per the 30/70 rule hinges mostly on a good antenna.

Sound Card Disconnecting during Transmit

One interesting problem that during transmission, and in particular in digital mode where a sound card is required such as via WSJT-X, it might so happen that the sound card disconnects from the computer, sometimes even requiring the sound card to be reconnected, the radio reset or power cycled. From observation what happens is that after a certain transmit power threshold, the sound card tends to disconnect from the computer, and many times with the transmit power being well below the nominal or selected transmit power.

The most common problem is the lack of a counterpoise or, a counterpoise that shares ground with the radio ground that leads to RF flowing over the outer shield of the antenna cable, right back to the radio and creating interference with the radio and other electronics.

Here are some solutions in order to remedy the situation:

add a counterpoise wire,
add counterpoise radials to the antenna,
add an RFi choke balun to the antenna,
use a toroid such as FT-240-43 with either 12 or 17 wraps of the antenna around it to reduce RFi,
ensure that the antenna ground (outer wire, sheath, connector metal) does not share a common ground with the radio common ground

Antenna Voltage Blocking Capacitor

In general, any capacitor placed in-line with an antenna will provide protection from voltage flowing into the radio such that current can be injected into the antenna, for any reason.

For example, given the MFJ-1925 box that can be used to control an ATAS-120 motorized antenna, the immediate line that feeds into the radio is in series with three $10nF$ capacitors in parallel through which either $8V$ or $12V$ will flow unwillingly due to the voltage being on the same line that controls the ATAS antenna.

As the simulator calculated, even though $8V$ is fed into the array of capacitors, only $24nV$ flow into the radio, which is more than negligible. Even though not illustrated, feeding $12V$ to the capacitor array, reduces the voltage to $36nV$ which is still negligible.