I am a simpleton can just about wire up a PL259 plug and don't know that much about electrics but always wondered what do the coils do in the rigs why do they need them for?

I'm afraid you'll have to learn a little bit of basic electronics (I'll be gentle, and try to make it easy to understand):

In electronics, we have three basic "passive" components: Resistors, Capacitors and Inductors ("coils"). We have two basic kinds of electric current - DC and AC. DC is what comes out of a battery, and is pretty constant. AC is electic current that varies all the time - and RF is just a higher frequency kind of AC.

Resistors have the property of "resistance" (made by passing the electricity through a material that conducts less well than metal). They limit the flow of electricity (AC or DC) through them by turning the power into heat. Resistance is measured in Ohms, and the passage of current is described by "Ohm's Law" (named after Mr. Ohm). Current passing through a resistor is linearly proportional to the applied voltage and the "value" of the resistor in Ohms (Ω):

V = I × R I = V ÷ R and R = V ÷ I where

R is Resistance in Ohms (Ω) V is Voltage in Volts (V) I is Current in Amps (A)

Capacitors consist of a couple of metal plates separated by a non-conducing material (or sometimes just air). DC electricity is blocked - there's no way for it to get through, but AC can pass by means os creating and collapsing an electrostatic field around one plate of the device. This field is collected by the adjacent (although disconnected) other plate and AC current can flow. Capacitors have their own analogue of resistance (also measured in Ohms) called Capacitive Reactance (XC). The relevant formula shows that XC is inversely proportional to the value of capacitance in Farads (F) and the frequency of the AC in Hertz (Hz). Where Xc is Capacitive Reactance (in Ohms), f is frequency )in Hz) and C is the Capacitance (in Farads)

Inductors (coils) are just coils of wire, sometimes wound around some other material, sometimes free-standing in air. The number of turns, the thickness of the wire, the type of material it's wound around and all the physical dimensions of a coil determine its Inductance (in Henrys).
They also exhibit Reactance, because they use up electrical energy by constructing and collapsing magnetic fields! .

Where Xl is Inductive Reactance (in Ohms), f is frequency (in Hz) and L is the Inductance (in Henrys).

[Incidentally, I is used for current because C had already been used for "Charge" (in Coulombs) and L is used for indictance because I had already been used....]

You will see that Inductive reactance is linearly proportional to Inductance, and capacitive reactance is inversely proportional to Capacitance.

If you connect a coil and a capacitor in parallel, the frequency at which their two Reactances match is the point of Resonance. That's how you tune things!

I hope that this goes some way to explaining what those curly bits of wire are doing inside your rigs!

FM listener wrote: ↑Thu Feb 13, 2025 9:24 pm I am a simpleton can just about wire up a PL259 plug and don't know that much about electrics but always wondered what do the coils do in the rigs why do they need them for?

They serve 2 purposes, they ensure that the specific things the main output transistor expects for the best efficiency to produce optimum watts, they do this by being a certain internal diameter and how many winds on the coil depending on the output frequency of the circuit. The other main benefit and purpose of the coils is that if set precise it removes the spurious signals out of the signal thus reducing the harmonics dirty signals.

That's in an easy to understand without the technical blurb Albert said which is absolutely what you should begin with learning about if you'd like to expand your knowledge of electronics then RF circuits which are not for the beginner to electronic circuitry.

Lee - The above is just 'O'-Level physics! If you want to start to consider the actual electronics side of the things, you're going to have to learn some fairly abstruse mathematics (including some more advanced algebra and calculus).

To the original poster - Inductors are used for several functions inside rigs. Tuned circuits (to determine frequency) use the above formulae, filters (to prevent radiation of the multiple of your transmit frequency that are always generated by even the very best-designed rigs) use a different set of formulae, but the reactances of the capacitors and inductors (described above) are crucial in their design.

Another function of inductors is to keep the RF energy where we want it, and nowhere else. Because a higher value inductor will exhibit a very high Xl at radio frequencies whilst still passing DC (for power to the active components like transistors), they are used to keep the RF energy from leaking on to the supply lines. It's quite common to find a small ferrite bead inductor on the inputs of audio equipment, preventing RF getting inside and wreaking havoc making hums, buzzes and other noises you really don't want!.

Because inductors have Reactance, they can be used to "match" a power transistor to its load (the aeriial) in an optimum fashion. Typically, a higher power amplifier stage mught have an output impedance of just a few Ohms. Traditionally, (and with good reasons) feedlines and aerials are generally designed to match to 75Ω or (more usually) 50Ω. The connectors joining the coax to the transmitter will also be characterised for 50Ω, in an effort to make the join electrically "invisible".

At the aerial, it's quite common to find coil and capacitor networks in the connection boxes to "match" the 50Ω feeder to the aerial which might exhibit a rather different load impedance.

As you can see, those little wiggly bits of wire are crucial to radio engineering!

Along with trim pots they are for increasing your power output.

That's the wrong answer by the way. Long ago before I even knew what a balun was I had a VFO rig which was quite stable and the PA board had coils and 2 x of the larger yellow varicaps. I used to twiddle them around to get the power to go upwards on my power meter.

The trouble is the dipole was 75Ohms and whilst cut correctly probably had common mode current flowing on the coax outer side. The coax was 50Ohms so already there was a wrong match and of course cmc may also affect the swr. It was reading about 2.3;1 and would not come down so was not ideal but went ahead anyway. I still did 20miles and was just a 15w rig.

Albert wrote a great description for which I am thankful also.

Radio is pretty complex. It certainly needs a mathematics mind to get into it at electronics level. Just to a put a small station on without blowing rigs all the time takes a little knowledge, but not too much a bit power concepts, standing wave radio, cutting a dipole or GPA and using coax feeds, soldering a few connectors on etc. Even with all the maths as I gather there is a lot experiments and practical techniques and individual component knowledge to understand on top of it all. And that is to make pre existing stuff, the real clever stuff happens at design level and you probably need creativity and a bunch of good 'what if ?' ideas, some won't work and I bet some you can work through with technical solutions and then some do.

Many people are interested and fascinated but the maths and many concepts becomes a block for most.

One thing I will say, for most people we might think that we cannot do something maybe confidence is low or out of our comfort level, but if we actually push our selves and be encouraged by a few people who know more than ourselves you can often be more than you think so bear it in mind.

The maths really isn't too difficult, but the real problem (particularly in the UK) is that it's very poorly taught. Many schoolchildren say that they "hate" or "don't get" maths - simply because their teachers are so poor. I was lucky - I grew up in a household with engineers and medics around me, and they all had a good grasp of mathematics. I was able to pass a UK "A"-Level maths at 8 years old!

In engineering, mathematics is just another tool - as important as a screwdriver or soldering iron. Unfortunately, pirate radio has a whole lot of clueless solder-jockeys who don't really understand what they're doing, and simply copy the work of others. I can't count the number of rigs I've seen with my old circuits from the 1970s and 1980s! One little trick I did was to specify some slightly unusual resistor and capacitor values (for no other reason than to prove my thesis!) and was astonished to see just how many solder-jockeys slavishly followed those values without any understanding of what they were doing.....

Another trick I used was to include 2-wire "bidirectional" green LEDs in my clipper stages in limiters. The clueless just put in ordinary green LEDs and couldn't understand why their builds didn't work properly! The choice of the green bidirectional parts was because - very conveniently - they went into conduction at exactly PPM 6, effectively protecting against overshoots causing overdeviation.

In some respects it's frustrating to see so many copies of my work, though it's also quite flattering too, in a perverse sort of way!

Their there separate different parts of the circuit, not much good without a coupling cap, to stop the power rails becoming part of the circuit, it wouldn’t work at all. The size of the coil, c capacitance are dependent on frequency, higher frequency lower the pf. The coils in the output are there to filter the out of band harmonics, for higher power thicker wire needs to be used.

FMEnjoyer wrote: ↑Sat Feb 15, 2025 10:14 am The trouble is the dipole was 75Ohms and whilst cut correctly probably had common mode current flowing on the coax outer side. The coax was 50Ohms so already there was a wrong match and of course cmc may also affect the swr. It was reading about 2.3;1 and would not come down so was not ideal but went ahead anyway. I still did 20miles and was just a 15w rig.

I feel out of my depth seem often to be talking about high power commercial operations, the impedence comes more into play with more power, maximun power transfer happens at 50 ohms, 75 ohms lower loss, it is possible to get low power transmitters of a few watts to work with 75 ohms. Heating of the output stage, depends on matching, the design of the transmitter, what wattage its meant to operate at. If you know what you're doing its better to put your own rf amplifier together filter, you will know the intended purpose.

I have microwave frequency PAs that match their hard-line outputs at 20Ω! I have valved PAs that are designed to match into 200Ω.

50Ω and 75Ω were just chosen as common standards, largely for the sake of convenience. TV coax (from the Yagi on the roof down to your TV) is characterised for 75Ω.

Incidentally, a "natural" match for a 1 Watt transmitter running from a 12.25V supply is almost exactly 75Ω, which was very convenient for my Band IV link gear! I used to use little 1 Watt synthesised rigs into 75Ω coax and a TX aerial, and the receivers were based on a TV tunerhead (75Ω input of course) using a TV Yagi. The advantages were that very high gain directional aerials were available in every DIY store for very little money, the coax was cheap and very low-loss, and a TV aerial is never out of place on a rooftop! I had (relatively) low-powered links that would work over several miles with ease!