RF Cafe Software

RF Cascade Workbook 2005 - RF Cafe
RF Cascade Workbook

Calculator Workbook
RF Workbench
Smith Chart™ for Visio
Smith Chart™ for Excel
RF & EE Symbols Word
RF Stencils for Visio

About RF Cafe

Kirt Blattenberger - RF Cafe WebmasterCopyright
1996 - 2022
Kirt Blattenberger,

RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The Internet was still largely an unknown entity at the time and not much was available in the form of WYSIWYG ...

All trademarks, copyrights, patents, and other rights of ownership to images and text used on the RF Cafe website are hereby acknowledged.

My Hobby Website:

Try Using SEARCH
to Find What You Need. 
There are 1,000s of Pages Indexed on RF Cafe !

Electronics World Cover,TOC,and list of posted Popular Electronics articles QST Radio & TV News Radio-Craft Radio-Electronics Short Wave Craft Wireless World About RF Cafe RF Cafe Homepage RF Cafe in Morse Code Google Search of RF Cafe website Sitemap Electronics Equations Mathematics Equations Equations physics Manufacturers & distributors Engineer Jobs Twitter LinkedIn Crosswords Engineering Humor Kirt's Cogitations Engineering Event Calendar RF Engineering Quizzes AN/MPN-14 Radar 5CCG Notable Quotes App Notes Calculators Education Magazines Software,T-Shirts,Coffee Mugs Articles - submitted by RF Cafe visitors Simulators Technical Writings RF Cafe Archives Test Notes RF Cascade Workbook RF Stencils for Visio Shapes for Word Thank you for visiting RF Cafe!

One Problem in Choosing Test Leads
July 1953 QST Article

July 1953 QST

July 1953 QST Cover - RF CafeTable of Contents

These articles are scanned and OCRed from old editions of the ARRL's QST magazine. Here is a list of the QST articles I have already posted. All copyrights are hereby acknowledged.

Authors Cohen and Hessinger warn about the need to consider the capacitive loading effects of shielded and closely-space test leads when measuring other than direct current or very low audio or line frequencies. Lead capacitance is especially likely to affect measured values when the frequency is high and/or the source and load impedances are high. As was common in the day, capacitance units of μμfd (micro-micro farads = 10-6 x 10-6 = 10-12 F) are cited, which is equivalent to units of pF (10-12 F).

One Problem in Choosing Test Leads

By George S. Cohen,* W8HTI, and Richard W. Hessinger*

Every audio enthusiast, amateur radio operator, and electronic technician finds it necessary at one time or another to make some sort of electrical measurement. Many of these measurements involve d.c. Bully for the man who has only this type of measurement to make. But there are those of us who are not quite so fortunate, for we must make many measurements on a.c. equipment. (This "a.c. equipment" means anything other than d.c., and so involves a.f. and r.f. as well as commercial power frequencies.)

Electrical circuit of any generator with an internal impedance - RF Cafe

Fig. 1 - The electrical circuit of any generator with an internal impedance Zi working into a load ZL. A typical example is a vacuum tube, where Zi becomes the plate resistance and ZL the load impedance.

Any practical voltmeter can be represented by the meter - RF Cafe

Fig. 2 - Any practical voltmeter can be represented by the meter M and Zi the impedance of the meter and its leads. Unless Zi is considerably higher than ZL, the indicated voltage will be lower than V.

When Zi and ZL have the values indicated - RF Cafe

Fig. 3 - When Zi and ZL have the values indicated, V = 0.375 EG.

If ZS is equal to ZL - RF Cafe

Fig. 4 - If ZS is equal to ZL, the indicated value of V will be 0.23 EG.

 - RF Cafe

Table I

All of these measurements require some type of connecting leads from the energy source to the measuring instrument. In the course of making many measurements it was found that a source of difficulty, in one instance, was in finding satisfactory leads for the particular job at hand. Although the measurements of current and voltage present similar, if not identical problems, we will consider voltage only.

Let us consider the generator of Fig. 1 with an internal impedance Zi. This source is feeding a load with an impedance equal to ZL. This circuit is very general and, as an aid in visualizing the problem, the generator may be thought of as a voltage-amplifier stage in an audio amplifier where Zi is the plate resistance of the tube and ZL is the impedance the tube is working into.

If we assume that the generator is a pentode voltage amplifier and that Zi is, therefore, of the order of 0.5 megohm and that ZL is in the vicinity of 0.3 megohm, we will have a specific value of voltage, V, across the load.

If Zs is all capacitive reactance, as in this case, it will combine with a resistive ZL of 0.3 megohm to give a resultant 0.21 megohm and not the 0.15 megohm shown in Fig. 4.

Now, as is often required, let us measure the voltage across the load impedance ZL. This circuit is-shown in Fig. 2. M is the meter or measuring instrument and ZS is the combined impedance of the instrument and its attached leads.

The voltage V that is measured in Fig. 2 will be approximately equal to V of Fig. 1 only if the impedance ZS is ten or more times greater than ZL. This happens because the shunting effect of the impedance, ZS, combines in parallel with ZL to give a lower resultant value.

As an example, let us suppose that ZS is equal to ZL - 0.3 megohm in this case. The parallel combination of ZS and ZL would then be equal to 0.15 megohm. Fig. 3 shows that under the original conditions V would be 0.375 times the generated voltage, EG. With the effect of ZS the voltage across the combination would drop to 0.23 times EG, as in Fig. 4. The difference in voltage is made up by additional drop across the plate resistance of the tube.

Further, if this amplifier were operating at 2 kc. (a reasonable frequency within the range of most audio amplifiers) then the shunt capacity necessary to give ZS a value of 0.3 megohm would be 265 μμfd.1

A value of 265 μμfd. may seem very large and unlikely to be found in an ordinary pair of test leads. Recently, in testing a circuit, we found that our calculations and actual results didn't agree very closely and that started the usual search.

The culprit was soon found. A shielded multi-conductor cable exhibited a capacity of 225 μμfd. from leads to shield and 128 μμfd. between leads. After this discovery, many leads in the laboratory were measured. One of these was a 4 1/2-foot shielded test lead supplied with an oscilloscope made by a well-known manufacturer. The lead measured 220 μμfd. between shield and the center conductor. A compilation of the capacities measured between other types of leads is recorded in Table I. An examination of Table I shows that the experimenter must use good judgment in choosing an instrument lead for voltage measurement.

There is a second consideration in choosing a lead and that is noise pick-up of the cable. But we will not enlarge on this subject here.

There are, obviously, many more types of leads than are listed in Table 1. The manufacturers of these usually list the capacity per foot of each of the different types and other characteristics that should be considered in using these cables.

We can do no more than point out one of the pitfalls and hope that it has, or will at some later date, save you from excessive blood pressure and loss of a few precious hairs.

* Commonwealth Engineering Co. of Ohio, 1771 Springfield St., Dayton, Ohio.

1 Capacitance equation - RF Cafe



Posted May 9, 2016