# Tag Info

4

Remember the time-honored engineering adage: Better is the enemy of Good Enough. It is difficult to imagine any practical value to inserting a matching network between your generator or feedline and your antenna: 49.4 - j8.1 ohms equates to an SWR of less than 1.2:1 on a 50$\Omega$ line, already a very good match. Given the relatively large tolerances and ...

4

It is not an either/or situation. Impedance controlled traces make feedlines of a known characteristic impedance, just like most coax is 50 or 75 ohms, and ladder-line is often 300 or 450 ohms. As a rule of thumb, any trace or wire longer than 1/10th the wavelength has to be considered a transmission line, in which case you must know the characteristic ...

4

First you don't necessarily have to do anything. The SWR is below 3 for the bands and most transceivers can handle that OK. If you want to try to get the resonate points inside the bands you will need to lengthen the wire. it is usually a good idea to start with too much wire and trim the length to bring the resonance down, but in your case it look like the ...

4

The short answer to your question is, "Yes." Every change made to an antenna is likely to change the feedpoint impedance: lengths, diameters and configurations of driven and parasitic conductors; positions, values and parasitic characteristics of "loading" circuits; distance from ground and "nearby" conductive structures, including near-resonant feedlines; ...

3

Think of it first without the feedline: The half-wavelength of transmission line provides a 180 degree phase shift, and the (short) connection between the shield ends is "ground". That means it's at 0V relative to the environment, and also the voltage at the open ends of the coax center conductor will be equal in magnitude but opposite in sign relative to ...

3

Take three 1/4 wave sections of 75 ohm coaxial cable (let's say RG11) and solder together. You now have a 1/4 wave section of 25 ohm. Doing the math (25 * 25) / 12.5 = 50 ohm. Now use any 50 ohm coxial cable length to the radio. Ps: don't forget to take into account the 75 ohm cable speed factor to calculate the 1/4 wave section. Good luck. Luiz PY4ACP

3

Anything done to get the antenna impedance closer to 50 ohms (or whatever the design might call for) can be called an impedance match, loading coils included. However I would note: many loading coils aren't exactly equivalent to an inductor at the feedpoint. Quite often, the loading coil is not at the base of the antenna but rather somewhere in the middle. ...

3

Well, according to the spec tables in the manual the thing expects a 50 Ω antenna impedance. Mismatch means a loss of signal energy. How much exactly depends on the length of the 75 Ω transmission line – this makes a system much harder to design predictably. I wrote all the following (up to the next horizontal line), then stopped to wonder "what might ...

3

I think the problem you will run into is that people will want this to be plug and play. They may not have a directional coupler, or know or feel like to calibrate it and all that other stuff. Even if they do it, they may wonder if they did it right and if the reading will be accurate or not. I think you will need to handle this part in your design and ...

3

According to Vincent D. Matarrese's Master's thesis, "Tapered radio frequency transmission lines" (Portland State University, 1992), there is a long history of using tapered-impedance transmission lines in matching systems. It's much too big a topic to review in an SE post, but the thesis is loaded with references for exploration. The Delta match is an ...

3

An antenna that's Pretty short (0.08 wavelength), Not tuned, Mag-mounted to an inadequate counterpoise, Indoors with who-knows-how-much metal in its near field, could certainly have a 5:1 or worse SWR, which is enough to "peg the needle" on most meters I've seen.

2

Smith chart shows that 50 Ohm antenna (assumed perfect 50 Ohm real!) connected to 75 Ohm coax cable can result in impedances between 50 and 112.5 Ohm. SWR 1: 1.5. So this so far not mentioned possible solution in this thread may be usefull for your problem: for a single frequency, or for a small frequency band, the use of cable with a multiple of half ...

1

To add to Phil's answer: if your antenna, source and transmission line (the impedance controlled traces) are all of the same impedance, you don't need an impedance matching circuit. Most common impedance for RF applications is 50 ohms. The impedance matching circuits are only there to connect two systems with a different impedance to each other. Creating ...

1

A "loading coil" in series with its feedpoint terminals can offset the capacitive reactance of an electrically short set of radiating wires (where the jX term of the feedpoint impedance R -jX is zero ohms at the operating frequency). However the loading coil value and its physical location in the antenna system have much less affect on matching the R term ...

1

Yes. A vertical antenna element shorter than λ/4 at the required frequency would be capacitive. The inductive reactance of the loading coil at the base of the antenna would offset the capacitive reactance of the short antenna and make it resonant at the required frequency. Likewise with two loading coils at the feed point of a short dipole. A tuner would ...

1

LC filters make sense as they don't dissipate energy. Hm, that's kind of true (at least theoretically), but guess what: that energy that doesn't reach the antenna still has to go somewhere, and will eventually be converted to heat. In the case of LC filters, that might be in some ohmic resistances in components and traces, or in the amplifier output stage. ...

Only top voted, non community-wiki answers of a minimum length are eligible