Re: MFJ-269 Antenna/SWR/RF Analyzer

Tam/WB2TT wrote:

It's a high pass filter to reject the AM broadcast band. So, the two 1000 PF caps are in series. Am I missing something?

Sorry, I missed that you had made a T network rather than pi.

In general, a tee network substituted for a pi will have the same characteristics only at one frequency, but will have different transfer and/or impedance characteristics at other frequencies. So the substitution should be done with care if characteristics are important at more than one frequency.

In this case, though, if you make a tee network which has the same "half wave" characteristic as the pi at the design frequency, it'll have identical transfer characteristics (it's got the same filter response) and complementary impedance characteristics. That is, at frequencies where one network has an input impedance greater than 50 ohms, the other will have an impedance that's less, and the phase angles are the negatives of each other. And, luckily, the transformation is simple for this particular special case -- the T network reactances are also all the same and also equal to the Z0 of the "transmission line". So one is just as good as the other.

The HPF equivalent doesn't of course simulate a transmission line, although the impedance transformation though the filter is unity at the design frequency. Otherwise, it works in pretty much an opposite way from the LPF.

I need to correct and clarify a couple of points I made in my earlier posting.

The "half wave" lowpass filter simulates a half wavelength transmission line only at and near the design frequency (where the reactances are all the same). It doesn't do a very good job either above or below that frequency(*). The 7 MHz example terminated with 50 ohms will show an input impedance within 2 ohms magnitude and 2 degrees phase of 50 ohms between about 6.2 and 7.4 MHz, so it's good for the entire 40 meter band. But it will disturb measurements on lower bands. You should construct one for each band and, preferably, one for each general impedance level you expect to measure. A single one won't do for multiple bands as I implied.

(*)For a better general simulation of a *short* transmission line, use a ladder network with all the reactances equal to Z0 except the end components. For the end components, make the series L or shunt C half the value of the rest. (For example, the LPF I showed would have input and output shunt capacitors with reactance = 100 ohms, and remaining components with reactance = 50 ohms. A five-component tee type network would have input and output series inductors with reactance = 25 ohms, and the remaining components with reactance = 50 ohms.) This model improves -- in theory at least -- as more sections are added, being able to imitate longer and longer lines. In practice, imperfection in the components limits the quality of the approximation. But I don't think this is of particular interest in making analyzer measurements. The model I proposed is better for simulating a half wavelength line while providing filtering.

Roy Lewallen, W7EL
.

Relevant Pages

  • Re: Current across the antenna loading coil - from scratch
    ... bottom, around tens of ohms, to being high at the top, around thousands of ... response to technical argument or trying to go step by step. ... he "knows" I mean reactance. ... When you used "impedance", I couldn't guess and figure out if you meant ...
    (rec.radio.amateur.antenna)
  • Re: Antenna analyzers, opinions please...
    ... As you know this is 1/2 the impedance of a 1/2 wave ... dipole-70 ohms in free space. ... impedance below 40 ohms with little reactance should provide you with ... In that case, radial current can be significant at quite a distance from the antenna (as opposed to the exponential-looking decay you see in the current on radials buried in moist ground), making the system act more like a system of elevated radials. ...
    (rec.radio.amateur.antenna)
  • Re: Trying to find out what Ferrite material this is.
    ... Have you tried to explore self resonance of a coil..., invariably it runs into the same problem of inductive reactance going off scale way below the point at which self resonance bites in. ... When talking about ferrite or powdered iron cored inductors of reactance over 200 ohms, where mu is frequency dependent and flux dependent, I find the '259B nearly useless. ... For most toroidal inductors the impedance is closely proportional to the square of the number of turns, so the value of multi-turn inductors can be extrapolated with reasonable accuracy. ...
    (rec.radio.amateur.antenna)
  • Re: HF Gain Stepping RC Network Calculator
    ... The network in this spreadsheet is simply a combination of your series ... Column E is simply the reactive part of the impedance of the ... Gain Stepping RC Network Calculator", ... the pole and zero frequencies which can be used to directly sketch out ...
    (rec.audio.tubes)
  • Re: HF Gain Stepping RC Network Calculator
    ... The network in this spreadsheet is simply a combination of your series ... Column E is simply the reactive part of the impedance of the ... Then multiplying the numerator and denominator by j*w*Cs we get: ...
    (rec.audio.tubes)