Active Receiving Antennas

An active antenna is sometimes used for receiving purposes in instances where a normal antenna would be impossible to accommodate in a physical sense. Such an antenna is sometimes called an aperiodic antenna other people refer to it as an antenna booster.

Because modern receivers now cover such a broad frequency range it is also desirable to have a broad band antenna. In this tutorial I will attempt to address some of the theoretical justifications for using such a booster antenna.

It must be borne in mind at all times that an active receiving antenna is quite capable of introducing more severe problems into a receiving system than those which it is intended to solve.

Consider now some theoretical basics where we might compare a one metre long whip with a standard quarter wave antenna in the amateur 40 metre band. I simply selected the 40 metre band purely on whimsy and the principles could still apply at 80 metres, the a.m. radio band or even at long wave 175 Khz (1700 metres).

Conveniently a standard 1/4 wave antenna at 40 metres would measure approximately 10 metres in length. Assuming a distant signal, when received on that antenna, had a field strength of 10 uV per metre it would induce in our 1/4 wave antenna an open circuit voltage of 100 uV.  Again for convenience only we will assume a pipe diameter of 20 mm.

Now on our one metre whip the same signal would induce an open circuit voltage of 10 uV. Are you following me here? I haven't used any fancy maths yet but I'll introduce some useful formulae now. NOTE the terms "an open circuit voltage", that is VERY significant. Here we will assume a whip diameter of 3 mm.

Whip Antenna Capacitance

Fig 1.

Where Ca is the antenna capacitance (in pF), 'h' is the height and 'a' is the whip diameter, both measured in metres. NOTE the co-efficient 0.7353 becomes 0.615 if dimensions are measured in inches. Ca is an approximation, many other factors come into play.

Short Whip Antenna Radiation Resistance

Fig 2.

Where Rr is the short whip antenna radiation resistance, 'h' is the height and l is the wavelength. This formula assumes a short vertical whip over a perfectly conducting plane which does not occur in reality.

Open Circuit Voltage

Open circuit voltage is simply the electric field strength multiplied by the physical height, as one example a particular signal might have a field strength of say 10 uV per metre (10 uV/M) and if the antenna height was say 12 metres long this would give us an open circuit voltage of 120 uV.

Simply it's the multiplication of 10 uV/M times the length. Be quite clear on the topic of open circuit voltage because it becomes quite important to your continued understanding.

Visualise a vertical antenna sitting out in the yard, 12 metres tall and conveniently for us there is only one signal available, it has a strength of 10 uV/M  giving a total signal voltage on the antenna without any load attached of 120 uV. Close the eyes and think about it.

When a load is attached and we want to take some power from this antenna then that's when the fun starts. If you don't understand the concept of impedance then go back to my inelegant, but readily understandable analogy NOW because without it you're going to labour after this and probably miss the point.

Comparison of Antennas

The quarter wave antenna (with 100 uV open circuit voltage) is well known to have a radiation resistance of around 30 ohms (I've used 36 ohms in my sums below) and, disregarding matching considerations if terminated in a traditional 50 ohm load would form a voltage divider action as in fig. 3.  This is purely for illustrative and comparative purposes only, please understand that particular point.

Fig 3.

In practice of course we would use ground radials and utilize matching techniques. How is the reduction in open circuit voltage worked out? Well consider this:

Vout = Vin X [ RL  / ( Rr + RL )] or;

58 uV = 100 uV X [ 50 / ( 36 + 50 )]

Let's look at our miserable one metre long whip which might be a few millimetres in diameter, we'll say for this exercise it is 3 mm diameter.

Using the formula above we get a Ca of about 12 pF (in practice it would most likely be more). At a nominal frequency of 7.5 Mhz the 12 pF reactance becomes a rough impedance of 1768 ohms in series with an insignificant Rr of much less than 1 ohm.

Fig 4.

Consider again,

Vout = Vin X [ RL  / ( Z + RL )] or;

0.275 uV = 10 uV X [ 50 / ( 1768 + 50 )]

Essentially the 50 ohm load becomes almost a short circuit to the signal. Now I can assure you I have taken a considerable number of liberties here in the name of simplicity but the picture I have demonstrated is pretty much the real world situation.

In fact if you want to play around with receiving antennas I'd suggest you thoroughly digest this lesson as set out above because it should prove sobering (no pun intended).

Role of the Active Receiving Antenna

Now here is the general justification of an active antenna. If a high impedance load is connected to our one metre whip instead of the 50 ohm dead short then our calculations proceed as follows.

Assuming we use a field effect transistor as the amplifying device. The circuit configuration will be a source follower. A source follower exhibits high input impedance and relatively low output impedance. It also has a voltage LOSS.

This voltage loss (without going into a lot of theory) is about 10% BUT the power gain is almost infinite.

That statement will cause a great deal of confusion so consider this (purely hypothetical), if the input to our source follower is 10 uV  into an impedance of 100,000 ohms (nominal for illustrative purposes) the input power taken by the FET is E² / R  which by way of calculation is 1 X 10 ^-15 watts.

The output delivered to a 200 ohm (nominal for illustrative purposes) load would be 90% of the input voltage (10% loss) or 9 uV and E² / R  which by way of calculation is 4.05 X 10 ^-13 wattsor 405 times the input power or a power gain of 26 dB.

That is the role of the active device in this antenna. Simple!, well NO, there are quite a number of problems involved here. Firstly all FETS have some inherent input capacitance and this is one of the limiting problems. Assume a possible input capacitance of about 5 pF and a real world one meter whip having a capacitance of say 25 pF including holder, feed connection etc. Oh dear here comes that fink voltage divider action again.

The FET driving voltage is:

Vin / [ 1 + ( Ct / Ca )]

where Ct is the FET input capacitance (5 pF) and Ca is the antenna capacitance (all up 25 pF). If our one meter whip produces our former 10 uV open circuit voltage then the FET driving voltage is: 10 uV / [ 1 + ( 5 pF / 25 pF )] or 8.3 uV.

This reduction in voltage is nothing to become paranoid about because subsequent amplification will quite readily make up the difference. BTW the FET source follower configuration used here is sometimes considered to be an impedance converter.

Noise considerations

The first critical issue is one of S / N ratio. Assuming we are using a fairly good FET it should not degrade the overall noise performance of the receiving system. Below say 15 Mhz this is rarely an issue.

In the example of the quarter wave antenna and a 10 uV/M signal, external noise levels might be 1 uV/M.

These noise levels would include QRN (natural noise) and QRM (man made noise, such as my computer is presently doing to a nearby receiver).  The noise is a constant ratio compared to received signal.

Whether we use a full quarter wave antenna or a physically short whip, the signal level and the noise level are going to be both proportional to physical length. The only consideration is whether the output signal level is going to be below the inherent receiver noise.

Limitations

So far you might imagine we could get away with an active receiving antenna as small as 50 mm in height (by the way it has been experimentally built). Unfortunately several vexing problems jump in our way. Don't discard the idea of an active receiving antenna because of them but be aware of the potential limitations. Some of these are:

1.    Compromise of Receiver Dynamic Range:

Dynamic range might generally be described as the ratio of the level of strong out-of-band signals to the level of the weakest acceptable desired signal. An active receiving antenna is very broad band by nature and by virtue of its design. Indeed that often is the principle goal. Unfortunately it will also likely compromise the dynamic range of an otherwise excellent receiver because ALL signals present on the antenna are amplified equally.

I spoke earlier of signal levels of 10 uV/M, QRP-CW (low power morse code) enthusiasts wouldn't consider that much of a challenge. BUT with a broad band antenna don't be surprised to encounter signal levels of VOLTS per metre (not micro volts) from nearby transmitters especially strong stations in the A.M. radio band.

Many years ago I was fooling around with a basic crystal set tuned to a local A.M. station. I had a ferrite rod antenna which was tuned with a variable capacitor. NO external antenna was connected but I did have my high impedance oscilloscope attached to the ferrite rod. I was astounded to see a perfect A.M. signal of nearly 20V P/P.

2.    Cross Modulation Distortion:

This occurs when the modulation (e.g. music) of an unwanted strong signal is transferred to a wanted weak signal (e.g. voice). I only highlighted those particular examples to give you an extreme understanding.

3.    Inter modulation Distortion (IMD)

Receivers with many active devices, especially a receiver which has as the first active device an active receiving antenna, will frequently react in ways that do not always agree with theory. IMD is a complex problem.

In the case of an active receiving antenna you must also consider that the active device is capable of functioning as a mixer. I could give you pages of mathematics indicating trignomatic identities involving the production of a number of components with difference frequencies.

One definition (for our purposes) might be: "IMD - occurs in any non linear device (our FET) when driven by a complex signal having more than one frequency (our broad band antenna signals of - from 10 Khz up to 300 Mhz or more). The resultant signals (our output) become distorted".

As only one example, the possible effects of the A.M. radio band on an active antenna:

Signal (A) 900 Khz; Signal (B) 1500 Khz; - both at a field strength of 1000 uV/M (easily common) and finally our desired Signal (C) 3900 Khz - it's a lowly 10 uV/M signal.

A possible mixing action in our FET (they make good mixers) might be;

2 X Sig (A) + Sig (B) = Sig (C)  0r

( 2 X 1500) + 900 = 3900 Khz

considering the high levels of A and B and the low level of desired signal C what hope do you think you have recovering C signal? Hey and I've only mentioned two interferers and only ONE mathematical combination from around millions of available signals and a hell of a lot of possible mathematical combinations. Want to weep?

Ref: http://my.integritynet.com.au/purdic/activeantenna1.htm

Receiver design - the fundamentals

Among the first radio receivers ever constructed I suppose must have been the ever so humble crystal set. Just how many have been constructed over the years would be impossible to guess.

It would be fair to say millions of people, especially children had their first contact with electronic construction via the old crystal set.

Without going into a detailed history of radio it is fair to say the modern radio communications receiver (beyond the basic entertainment type) has evolved to the point all of the following characteristics must be considered at length when either purchasing or building a receiver. This discussion is confined to the type referred to as a "communications receiver"

These characteristics (and not in any particular order) are as follows: 

 

1. GENERAL

All receivers of the type being discussed here are for conveying information between 2 or more people but the description could include specialised receivers such as direction finding, radar etc.

2. INPUT CHARACTERISTICS

As silly as it may sound the first requirement of a receiver is to efficiently and with maximum voltage levels possible, transfer electromagnetic energy from the antenna to the input of the first stage of the receiver.

Well that's pretty basic isn't it?.

You would be surprised just how neglected this area becomes when people establish a receiving set up. How many listeners simply hang up as much wire as possible, cross fingers and hope for the best. If nothing is heard on a particular band it must therefore be assumed there is nothing on the air to hear.

That ain't necessarily so.

You could be missing hundreds of good signals!. Why?. Because of a haphazard approach to interfacing your receiver to the real world. Sometimes, and I am presently in this boat myself now, your location will not allow the best antenna set up possible. Maybe you live in an apartment or face some sort of restrictions on what you may be able to erect on the property where you live. Throwing your hands in the air and lowering a few metres of wire out the window is not a terribly scientific approach. No wonder you are likely doomed to disappointment.

You don't need to be a rocket scientist to establish a functional set-up. Certainly you must live within the constraints imposed upon you but you can always strive for the better mouse-trap.

The how-to's I will leave until later. The important thing to remember now is that no matter how classy your receiver is, you just might be choking off all those elusive signals BEFORE they get to the input of the receiver.

The professional receiver designer has no idea what you are going to attach to it. Therefore conventional wisdom dictates it be designed for a 50 ohm (nominal) input. Some receivers also offer an auxilliary 500 ohm input.

2. GAIN, SENSITIVITY AND NOISE FIGURE

Your communications receiver it is hoped will encounter and process a wide range of signals. It must be capable of handling these signals usefully without introducing problems of its own. Consider a signal emanating from your favourite s.w. commercial broadcaster some 10,000 miles (16,000 kM) away.

This signal may originate with a power level of 20 KiloWatts (20Kw). By the time it reaches the input of your receiver the level may only be 1uV (1 micro-volt). The signal has been attenuated (reduced) by 180 dB. That's a one followed by 18 zeros.

For you to usefully and comfortably hear this signal at the output of your speaker, at a quite modest level of say 250 mW (milli-watts), the receiver needs to amplify the signal by about 130 db or have a gain of 130 dB. Now that's a one followed by 13 zero's. I would estimate that about nearly half that gain would come from the audio amplifier section. This would mean about 70 dB of gain  needs to come from the preceding stages.

Now for the moment I am going to deal with an a.m. receiver here. The sensitivity is influenced by the receiver bandwidth so we will assume a bandwidth of 6 Khz. That theoretically means the receiver will not respond to those portions of a signal which are outside plus/minus 3 Khz from the carrier. e.g. a signal on 27.24 Mhz. A good receiver undergoing a test at that frequency would indicate a sensitivity of about 1.5 uV.

Noise Figure is somewhat nebulous and tends to mean different things to different people.

To dispense with any arguments I will quote in part (omitting the later heavy mathematics) Professor Ulrich Rohde from his book "Communications Receivers - Principles and Design" - P68 -ISBN 0-07-053570-1 

 

"Sensitivity measures depend upon specific signal characteristics. NF measures the effects of inherent receiver noise in a different manner.Essentially it compares the total receiver noise with the noise that would be present if the receiver generated no noise. This ratio is sometimes called the noise factor F, and when expressed in dB, the noise figure."

 - bold type is my emphasis alone. 

  

 

3. SELECTIVITY

This simply means the ability of the receiver to separate the signal you want from all the other signals. This selectivity must be sharp enough to differentiate from adjacent channels yet sufficiently wide enough to reproduce the signal at an acceptable fidelity.

Some would say 300 Hz is ideal for C.W. (morse code) while 6 Khz (6,000 Hz) is too wide for serious short wave listening. A T.V. Receiver has a bandwidth of around 7 Mhz (7,000,000 Hz) and F.M. Radio uses 200 Khz channel spacing.

Therefore the selectivity should be consistent with the type of signal you expect to encounter.

4. DYNAMIC RANGE

Here you faithful lecturer jumps on/off high horse.

Just as with noise figure this means different things to different people. Some manufacturers will even omit this figure altogether in their specifications and a lot of people active in radio have never even heard of it.

It is one of the most critical characteristics of a receiver.

It is quite important how it is defined. 

 

Dynamic Range could be defined as:

"The ability of a receiver to survive in the presence of strong signals."

But I feel it should be defined as:

"The ratio of the level of strong out-of-band signals to the level of the weakest acceptable desired signal. The level of strong signal must be such as to cause the weak signal to become unacceptable". 

 

Expressed another way, it means if we are just managing to listen to our favourite elusive signal from far, far away we don't want a nearby channel, occupied by some powerful transmitter situated close by, to swamp out our desired signal and take control of our receiver.

5. GAIN CONTROL

Harking back to our earlier signal of about 1 uV level. In practice this signal level will vary wildly from instant to instant for a variety of reasons but mainly because of the vagaries of propogation.

Obviously it would be unacceptable for the reproduction to vary wildly at the output of your speaker in sympathy with the varying signal input. Also we don't want to continue amplifying the desired signal if it is already a strong signal at our antenna. Hence the need for automatic gain control.

Ideally we would want a constant output from our receiver regardless of the signal level presented at the input. Gain control should generally be logarithmic in response and a range of 120 dB would be ideal. The time constants of the response (i.e. how fast it operates etc.) should depend on the mode of receiving e.g. C.W., S.S.B. or A.M. etc.

6. FREQUENCY ACCURACY AND STABILITY

We all know how difficult sometimes it is to locate a station on a cheap a.m. radio. With a quality communications receiver we should be able to set our frequency of reception with both accuracy and certainty. We should also be able to remain on frequency for any length of time without the need to unduly re-tune the receiver.

The present state-of-the-art is such that these properties are no longer (or should not be) a problem. Even the lower cost receivers offer exceptional accuracy and stability compared say to 20 years ago.

Ref: http://my.integritynet.com.au/purdic/rec_basics.html

FM Transmitter

Here is the schematic, PC board pattern, and parts placement for a low powered FM transmitter. The range of the transmitter when running at 9V is about 300 feet. Running it from 12V increases the range to about 400 feet. This transmitter should not be used as a room or telephone bug. 

 

 

C1           1              0.001uf Disc Capacitor   

C2           1              5.6pf Disc Capacitor        

C3,C4     2              10uf Electrolytic Capacitor           

C5           1              3-18pf Adjustable Cap  

R1           1              270 Ohm 1/8W Resistor 270 Ohm 1/4W Resistor

R2,R5,R6              3              4.7k 1/8W Resistor          4.7K 1/4W Resistor

R3           1              10k 1/8W Resistor            10K 1/4W Resistor

R4           1              100k 1/8W Resistor         100K 1/4W Resistor

Q1, Q2  2              2N2222A NPN Transistor               2N3904, NTE123A

L1, L2     2              5 Turn Air Core Coil         

MIC        1              Electret Microphone     

MISC     1              9V Battery Snap, PC Board, Wire For Antenna    

1. L1 and L2 are 5 turns of 28 AWG enamel coated magnet wire wound with a inside diameter of about 4mm. The inside of a ballpoint pen works well (the plastic tube that holds the ink). Remove the form after winding then install the coil on the circuit board, being careful not to bend it.
2. C5 is used for tuning. This transmitter operates on the normal broadcast frequencies (88-108MHz).
3. Q1 and Q2 can also be 2N3904 or something similar.
4. You can use 1/4 W resistors mounted vertically instead of 1/8 W resistors.
5. You may want to bypass the battery with a .01uf capacitor.
6. An antenna may not be required for operation.

ref: aaroncake.net/Circuits/fmtrans.asp

Induction Receiver

The induction receiver shown below is very sensitive and can serve a variety of purposes. It is excellent for tracing wiring behind walls, receiving audio from an induction transmitter, hearing lightning and other electric discharges, and monitoring a telephone or other device that produces an audio magnetic field ("telephone pickup coil").

The receiving coil could be a "telephone pickup coil" if available or a suitable coil from some other device. The coil in the prototype was salvaged from a surplus 24 volt relay. Actually, two relays were needed since the first was destroyed in the attempt to remove the surrounding metal so that a single solenoid remained. Epoxy putty was used to secure the thin wires and the whole operation was a bit of a challenge. A reed relay coil will give reduced sensitivity but would be much easier to use. The experimentally inclined might try increasing the inductance of a reed relay by replacing the reed switch with soft iron. Avoid shielded inductors or inductors with iron pole pieces designed to concentrate the magnetic field in a small area or confine it completely (as in a relay or transformer) unless you can remove the iron. The resulting coil should be a simple solenoid like wire wrapped around a nail. Don't try to wind your own - it takes too many turns. Evaluate several coils simply by listening. Coils with too little inductance will sound "tinny" with poor low frequency response and other coils will sound muffled, especially larger iron core coils. This prototype was tested with a large 100 mH air core coil with superb results but the 2 inch diameter was just too big for this application.

The other components are not particularly critical. The 2N4401 can be just about any NPN general purpose small-signal transistor. The TL431 is a shunt voltage regulator but it is being used as an audio amplifier in this circuit. In fact, the whole device is nothing more than a low noise, high gain audio amplifier with a pickup coil connected to the input and other amplifiers will work equally well.

The circuit is built into a 8 mm cassette box with the power switch and earphone jack in the back. The circuit board is a piece of pink countertop laminate which looks good against the violet hue of the cassette box. The battery fits nicely into the box and a piece of foam fills in the remaining space. These video cassette boxes make nice project boxes, unlike audio cassette boxes which are too flimsy.

When you first turn on the unit you will probably hear a lot of buzzing from the wiring in the room. Rotate the receiver in a horizontal plane to find a "null" where the hum is minimal. If you can get a reasonable null, you should be able to hear distant lightning crackles or other magnetic noises. If you cannot get a null then go outside away from the building. Try holding the coil near electronic devices like your computer monitor, telephone (when in use), cell phone readout, etc. You can trace power wires behind a wall or ceiling by listening for a sharp increase in hum as the coil passes near the wire. Make sure that current is flowing in the wires to be traced by turning on a lamp or other appliance. (Here is an experiment to try: Build a line voltage lamp flasher that can be connected to the circuit to be traced. The desired wire will now have an on and off buzz - buzz sound that will be easy to distinguish. I wonder if you could even identify a specific breaker or fuse?)

Other wires can be traced if they are carrying alternating current in the audio range or a signal generator can be connected to produce the current. Connect the generator to the wire to be traced and connect the generator's "ground" to the house wiring ground. Also ground the far end of the wire you are tracing so that current flows in the wire. This ground connection can also just be a temporary wire laying on the floor running from the generator ground to the far end of the wire you wish to trace.

For the ambitious: try wrapping one or two turns of wire around the whole house and connect the loop to the output of an audio power amplifier (one channel of a stereo should work). Add a 4 ohm, high wattage resistor in series to protect the amplifier. You should be able to pick up the magnetic field fairly easily anywhere within the loop with the power amplifier supplying just a few watts of power.

ref: techlib.com/electronics/induction.html

Vertical ground plane antenna

For higher frequencies, a resonant antenna becomes feasible. For example, Fig. 8 shows a simple vertical ground-plane antenna which connects directly to 50 ohm coaxial cable without a loading coil or matching network.

Using the equations shown, a 49 MHz antenna would have a vertical element 57 inches long and ground elements 59 inches long. The vertical element simply connects to the center conductor of the coax and the ground elements connect to the coax braid. The elements may be mounted on a small square of phenolic, fiberglass, or other weatherproof board material. Try not to let dissimilar metals come in contact or, if they must, coat the contact area with silicone rubber. One simple approach is to make the whole affair from PVC pipe with copper wire or tubing on the inside. It is often desirable to have a fixed-frequency antenna with directionality for monitoring a particular station or for installing on an antenna rotator. For example, if you live within a mile or two of a fast food restaurant you can probably pick up the little wireless microphones they use to take orders. You are probably wondering why anyone would want to pick up those signals (which are around 33 MHz). Hmmm. Well, it would be a challenge. Or, how about building a dedicated antenna to receive a distant weather transmitter instead. Or the police in a neighboring town, or a remote airport. Those sound a little better. (When my kids were small I thought of making a tricycle "drive-up" window with real audio from the local fast-food restaurant - never got around to it...) The point is that a directional antenna will give greatly improved performance for any of the signals on your scanner. Multi-element yagi antennas are a good choice for single frequency reception and log-periodic antennas give excellent multi-band reception. The construction of these antennas can prove difficult and purchasing a factory assembled unit is usually a preferable approach. A three-element yagi is not overly difficult for the more experienced hobbyist and several design references are easily found on the internet. A search using "3-element yagi" turned up nearly 600 hits including excellent design articles and commercial sources.

ref: techlib.com/electronics/antennas.html

AM Band Antennas

A good AM Band antenna can be a simple long-wire strung between two trees or across the top of the roof. Even a modest length wire will give your receiver greatly improved reception with less static because the signal pickup is occurring some distance from the interference generating appliances in the house. An insulator mounted high in a tree so that the wire has a large vertical rise will give great results. Mount the antenna as high and as far from the house as practical. Use a good quality ceramic insulator for holding the wire and add a commercial lightning arrestor where the antenna meets the house (Fig.1). Ceramic insulators are available with built-in wood screws and can be screwed into a tree or the wood parts of the house by hand. The wire may be tied to the far insulator as shown but the wire will stretch with time and require adjustment. A "trick" is to pass the wire through the insulator and fasten a fishing weight to the end so that the weight hangs a few inches below the insulator. As the tree sways in the breeze, the weight will move up and down and the wire will remain straight! (I must confess that my long wire antenna is made with insulated wire thrown over a branch with a rock tied to the end... Who has time to do it right? I do have an arrestor, however!) A good place for the arrestor is directly above the point where the water line enters the house. Run a heavy gauge ground wire straight down to the water pipe and attach it with a brass grounding clamp (assuming a copper water pipe - always use compatible materials or corrosion will result.). This connection also makes an excellent ground for the receiver. A shielded cable lead-in wire can give improved results when the residence has unusually noisy appliances. If you do not have an AM radio with a coaxial antenna jack then consider using an auto radio. Auto radios are well shielded to prevent ignition noise from interfering with reception and all that is needed to make a superior receiver for the home is a 12 volt power supply and a speaker. Inexpensive AM car radios are quite common, being discarded for fancy stereo upgrades and even the cheapest car receiver will outperform most home radios.

Fig. 2 shows how to add a loading inductor to the antenna in the event that a little more signal strength is desired. Most long wire antennas will be considerably less than 1/4 wavelength at AM band frequencies and behave as though a small capacitor is connected in series. The inductor resonates with this capacity and will increase the signal strength significantly. The required inductance range is from about 200 microhenry at the high end of the band to about 2 millihenry at the bottom end of the band for a 20 foot antenna.

Fig. 3 shows how to build a programmable loading coil using a 4.5 inch PVC coupling (found in the plumbing supply area of the local home improvement store) and 22 gauge insulated wire. The coil is wound with 100 turns with taps brought out every 10 turns by twisting a little loop in the wire The total inductance of this inductor is about 1 millihenry so short antennas may need more turns for the lower frequencies.

.

ref: techlib.com/electronics/antennas.html