Finding antenna impedance without a analyzer

Discussion in 'General Technical Questions and Answers' started by N5FOG, Jan 6, 2010.

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  1. N5FOG

    N5FOG Ham Member QRZ Page

    I recently bought a Palstar ZM30 antenna analyzer and was using it to measure my inverted L so I can build a matching network for 160 and to tune my bug catcher (which it really made a breeze) and I started to wonder how feed point impedance (both R and J+/-) where measured before the days of analyzers and EZNEC.

    So how did you guys find the feed point impedance both resistive and reactive (capacitive and inductive) of an antenna before the days of antenna analyzers ??


    FOG
     
    Last edited: Jan 6, 2010
  2. WB2WIK

    WB2WIK Platinum Subscriber Platinum Subscriber QRZ Page

    There were G-R Admittance Bridges since the 1950s.

    They required a separate signal source and metering system but performed largely the same function as a modern day analyzer.

    Back in the early days, antenna feedpoint impedance wasn't very important to know (and most had absolutely no idea what it was) because as long as you could efficiently transfer power into it, that's all you needed to do. That was accomplished with matching networks, just as it is today.

    It doesn't matter what my antenna impedance is if I can efficiently match it.:p

    Before coaxial cable was available (it only came into use in the 1940s, and became more commonly available after WW2), nobody cared about any of this, and they didn't need to.
     
  3. SM0AOM

    SM0AOM Ham Member QRZ Page

    Duplicate post
     
    Last edited: Jan 6, 2010
  4. SM0AOM

    SM0AOM Ham Member QRZ Page

    Those who really needed to know the antenna impedances usually first calculated them using (quite complicated) mathematical relations.
    For commonly used antenna structures impedance data was tabulated in engineering textbooks.

    It was not until Schelkunoff devised calculation methods in the mid 40's
    which could be grasped by the average radio engineer that individual calculations became commonplace.

    For verifying the calculations engineers mostly used bridges of various kinds.
    The probably best known is the GR 916 impedance bridge of 1930/40's vintage. Before the introduction of these bridges, engineers usually "jury-rigged" a suitable circuit.

    Amateurs were introduced to impedance measurements primarily by the "Antennascope", a resistive bridge circuit that was described in CQ Magazine by W2AEF shortly after the war.

    73/

    Karl-Arne
    SM0AOM
     
  5. G3TXQ

    G3TXQ Ham Member QRZ Page

    In the mid 1970s I used a Noise Bridge and an HP41 calculator programmed to tell me the feedpoint impedance, given a bridge measurement at the end of the feedline:

    [​IMG]

    Steve G3TXQ
     
  6. CT2FZI

    CT2FZI Ham Member QRZ Page

    I like this forum because you never stop learning.

    And learning from the past experiences led to where we are today.

    Thank you for your sharing :)

    Luis CT2FZI
     
  7. W5DXP

    W5DXP Ham Member QRZ Page

    James Clerk Maxwell himself used impedance bridges with one bridge named after him. He died in 1879. Such impedance bridges were described in the first ARRL The radio amateur's handbook that I bought for $3.50 in the early '50s. The "antenna analyzer" function is more than a century old, an integration, downsizing, and repackaging of earlier component functions.
     
  8. SM0AOM

    SM0AOM Ham Member QRZ Page

    It might be added that the automatic vector analyzer which made a revolution in microwave and RF engineering in the early 70's also consists of one or more bridges, which are arranged so narrowband tuned receivers can respond to both the magnitude and angle of the reflection coefficient.

    No "mean feat" using late 60's technology...

    73/

    Karl-Arne
    SM0AOM
     
    Last edited: Jan 6, 2010
  9. WB2UAQ

    WB2UAQ Ham Member QRZ Page

    I'd also like to add my 2 cents:)
    First building homebrewed impedance measuring equip and then collecting commercial equipment has been a much greater part of ham radio for me since the mid 80's than getting on the air. On the commercial side I have collected:
    HP (Booton Radio Corp) 250 RX meter (0.5 to 250 MHz) from HP's NJD built in 1976.
    GR 1602 (20 to 1500 MHz)
    GR 821A (0.5 to 30 MHz) (latest find last Sep and unit is dated 1949)
    GR 1608 (low freq 20 - 20 KHz
    HP 805 slotted line w/ HP415 SWR indicator
    GR null meters
    BRC 160 and BRC 190 Q meters
    Millen GDO with its impedance meter (the GDO is the RF source coupled with loops).

    Most of them, as Steve, WB2WIK mentioned, need a signal source and some kind of null meter. As Karl-Arne touched on, making a measurement was not easy and fast by any means. In almost all cases, for each measurement freq, some kind of preliminary calibration / set up it needed. As an example, the GR 1602 requires adjustment of the "admittance standard" and the 821A needs an initial balance and you often need a ball park impedance range and even the sign of X so you know where to set the fixed and variable caps.
    Sometimes you can't get a null right away if you guessed the wrong sign of X.

    Each instrument has characteristics that makes it better or worse than some other unit. Some of them are good at low Z's and others at high Z's, etc..
    Several of them don;t have RF connectors on them. I have the type N adapter for the 250RX meter which I think is pretty rare.

    In the end, a great deal is learned, by me anyway because I am slow on the up-take:) !

    My granddaughter who is just turned 3 yrs old looked at the GR 821A and asked me if it was a stove :) :)

    73, Pete, WB2UAQ
     
    Last edited: Jan 7, 2010
  10. VK2TIL

    VK2TIL Ham Member QRZ Page

    I recently expressed my envy of UAQ's fine collection.

    Those instruments are some of the most beautiful products of the electro-mechanical era of test instruments. They combine ingenious electrical engineering with innovative mechanical design. The final work of the machinists is as good as any Swiss watch; that appeals to me as a lover of fine machining.

    All bridges descend from the discovery of the principle by Christie & Wheatsone; it’s somewhat analogous to Yagi & Uda in that it was Christie who made the discovery and Wheatstone who publicised it so that the bridge is commonly called the Wheatstone bridge.

    Bridges resemble oscillators in that the various varieties are composed of essentially the same elements; oscillators have a tuned circuit and an amplifier whilst bridges have resistance and reactance. Like oscillators, bridges are identified by the name of their inventor; we have Colpitts, Hartley, Vackar et al in the world of oscillators and Wheatstone, Maxwell, Schering et al in the bridge world.

    This does not explain the Universal Bridge; no, it’s not named after Trevor Universal. A Universal Bridge is designed so that its various variables can be connected in different ways depending on the circumstances; because this increases parasitics the UB is not usually used at RF.

    It's sometimes difficult to discern a bridge structure in very complex bridges but some thinking and re-drawing of the schematic usually produces a "light-bulb moment".

    There are two kinds of bridges; the first kind is well-represented in UAQ's collection. In this kind, the device-under-test (DUT) is connected to the Unknown port and the variable arm of the bridge is adjusted until there is zero voltage across the detector terminals.

    The value of the variable arm then equals the value of the DUT.

    The Millen bridge is a resistive bridge; the same as a Christie-Wheatstone bridge. It does a very good job of measuring pure resistance but the detector null becomes broader if there is reactance in the DUT.

    The more-complex bridges have variable reactances as well as variable resistances so that each can be adjusted. These variables are arranged differently depending on the use to which the bridge is to be put.

    I think that the “analyzer age” began when experimenters realized that information about the DUT could be obtained from the voltage (scalar) or voltage/phase (vector) across the detector port of a bridge.

    This is what I call the second kind of bridge.

    If we make a bridge in which all necessary elements (RF source, bridge resistors & detector) are the same resistive impedance (50 & 75 ohms are common) we find that the voltage & angle across the detector port is directly-related to the complex reflection co-efficient of the DUT. Reflection co-efficient is a fundamental property; others such as VSWR and RL are mathematically related to RC.

    If we know complex RC we know a lot about the DUT, especially if we have a Smith chart.

    I think that this is how a modern antenna analyser works; the Smith chart is replaced by computing power. I also suspect that some are using RF-IV techniques to increase accuracy.

    The return-loss bridge is simple to make; it has no variable elements. Care in construction & component choice can make homebrew RLBs useable well into the multi-GHz range.

    Here is a great photo that might be said to represent a mid-60s antenna analyser;

    [​IMG] [​IMG]


    All the elements are present; the only difference is that he is using a dual directional coupler to separate incident & reflected waves.

    I am the proud owner of an HP8405A VV and I can attest to it being a very fine instrument.

    Even the computer is there; note the large circular "slide-rule". These were specially made with Smith chart scales to allow quick calculation of the required parameters.

    And we know that this man was an engineer; note the pocket-protector. http://www.microwaves101.com/encyclopedia/pocket_protector.cfm

    So that's some background for those interested; it's necessarily incomplete as entire books have been written on the subject of bridges.
     
    Last edited: Jan 8, 2010
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