TERMINOLOGY & KEY DISCUSSION POINTS ON ANTENNA SYSTEMS:
 This is an info paper dealing with

several definitions & factors bearing on antenna design, construction, and installation. Because the info runs on

for several pages, I am providing it to you as an email attachment before we hold the September 2018 session

on “antennas”. You may choose to keep it, print it out, or discard it as you see fit At Thursday’s session we will

focus on fairly simple wire HF antenna designs that might be suitable for a new HF “ham shack.”

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ANTENNA: a device that converts electric currents into electromagnetic waves and vice versa. It can be

considered a “complex resistive-inductive-capacitive (RLC) network”. The antenna is a conductive pathway

erected above ground serving as a connected, conductive pathway running down to our transceiver

circuitry. The antenna system is one of the most important features of your radio operation & one that you

can greatly influence, optimize. We will discuss some basic antenna designs in the Thursday session.

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IMPEDANCE (Z) = a general term characterizing the resistance to alternating current flow by an antenna

system. Z involves the inherent resistivity of the “wire” and two forms of signal transformation caused by

capacitance and inductance. We want to adjust Z so that it best “matches” our antenna to our feed line

to our transceiver. If all three factors of resistance/reactance are adjusted to an optimal point, we will

achieve maximum efficiency in sending & receiving signals.

 
calculated mathematically or graphically

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THREE KEY FACTORS BEARING ON IMPEDANCE:

RESISTANCE: the “ohmic resistance” (measured in ohms - Ω} to the flow of electrons (AC or DC)

because of the inherent qualities of the metallic conductors themselves. This factor is something we can

only marginally influence in designing & building our antenna…..we basically have to accept the conditions,

the values our conductive wires have.

REACTANCE: the retardation, the “resistance” to electron flow of an AC current presented by the

capacitive and inductive characteristics of the pathway (conductor). Also measured in ohms {Ω}.

Represents a power that is stored (retarded) in the near RF-field of the antenna.

INDUCTIVE REACTANCE (XL) = if the wire of our antenna is a bit longer than optimal for a given

frequency, inductive reactance increases.

CAPACITIVE REACTANCE (Xc) = if the wire is a bit too short, capacitive reactance increases.

At some frequencies along a radio band for our particular antenna, “reactance” will appear as an

inductive reactance; at other frequencies along the band capacitive reactanc becomes a factor. What

we want to do is to try to get these two factors to cancel one another out .

At a specific frequency for a given length of antenna wire, both reactances can be made equal in

magnitude, but because they are opposite in influence they can be made to cancel one other. By canceling

the effects of these two forms of reactance, we can reduce all of these “resistances” to current flow down

to solely ohmic resistance…..the inherent ohmic resistance to current flow. At this specific frequency,

the impedance is purely resistive and the antenna is said to be resonant…….a condition or state where

XL and Xc cancel out one another, leaving the impedance solely based on the OHMIC RESISTANCE of

the conductor (the antenna wire).

We achieve resonance in our wire antenna by basically shortening or lengthening the antenna wire

lengths, a process called “trimming” or perhaps by employing devices like “matches” to bring about the

best balances to cause reactance to cancel out, leaving purely OHMIC RESISTANCE.. The “sweet spot” of

resonance can be affected by our choices of materials & lengths of feed lines and by the height & configuration

of the wire we use to build out antenna.

This is where using an antenna analyzer is valuable in analyzing our circuit to help us make decisions

on changes in the antenna. Using an analyzer is a topic deferred for a later class or discussion.

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FEED POINT IMPEDANCE : a calculation made of the impedance at the point where a feed line connects

to an antenna. usually measured as [Z = voltage ÷ current] at the feed point. Most radio circuitry used by

hams is pegged to a 50 ohm impedance at the feed point as being ideal. There are ways to modify Z at the

feed point using tuning devices or baluns; That is another topic deferred until a later day.

Impedance levels at potential feed points along a “dipole” antenna vary, with the antenna’s impedance

dropping to a low point at the center of a 1/2WL antenna & rising to very high levels at the two ends of the

wire. A 1/2 WL wire antenna is a pretty effective length of conductive wire that can effectively receive

incoming signals & send out our transmitted signals….for a particular frequency. . The radiation resistance

at the center feed point of a 1/2 WL dipole antenna in free space (isolated from anything conductive) is

about 73 Ω. BUT, this “feed point impedance level” is also affected by the antenna’s height above the ground

and whether or not it is running perfectly parallel to the ground. Height & angle of the antenna can affect the

actual impedance. Ideally we would want to gauge the impedance of our antenna at the precise point up in

the air where the redline attaches. But this is not really practical so we usually end up performing the

measurement at ground level at the end of the feed line leading into the shack. So in reality, we are gauging

the impedance of the antenna + feed-line working together.

If we achieve “resonance” for a particular frequency on our antenna wire, we also hope to achieve a

condition where this condition of “resonance” or nearly “resonance” extends to other frequencies above

& below our chosen frequency. This is where a measurement of something termed “standing wave ratio”

comes into play.

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VOLTAGE STANDING WAVE RATIO (VSWR):

The voltage standing wave ratio (VSWR) (often simply termed as SWR) is a measurement of how well

an antenna is matched to efficiently handle RF energy generated by a source. Desired impedance levels

for radio circuits is usually targeted at 50 Ω. It is usually measured, calculated by reading the voltage wave

that is headed toward the load (antenna) versus the voltage wave that is reflected back from the load

(towards the transceiver). A perfect match will have a SWR of 1:1. This means that theoretically, 100%

of our transmission energy is being fed to the antenna, with zero reflected back. The higher the first

number of the SWR, the worse the match, and the more inefficient the system. A perfect match can rarely

ever be achieved, so we must be prepared to accept, to deal with an SWR slightly less than perfection.

In the case of a practical antenna SWR, a max of 2:1 can often be regarded as the higher SWR which

our transceiver can accept & deal with by itself. At this SWR, 88.9% of the energy sent to the antenna by

the transmitter is radiated into free space and 11.1% is either reflected back towards the source or lost as

heat” in the antenna or transmission line. And in receiving incoming signals, 88.9% of the captured

energy is ransferred to the receiver.

SWRs are often described using only the first number of the ratio, like “an SWR of 1” for a 1:1 ratio,

or “an SWR of 1.7” for a 1.7:1 ratio.

  Red curve reflects the SWR at several freqs,


Graphically displayed, an SWR plot for an antenna will likely show a “dip” where the SWR drops to a

low” point somewhere across a frequency span. The plot might be a shallow “dip” (as shown above) or

it could be a narrower sharply downward pointing “dip”. The effective “bandwidth” of a particular antenna

would be gauged by the width of this “dip” pattern. In this plot, the part to the left of the low point of the red

curve indicates a rising SWR value with capacitive reactance kicking in; the red curve to the right of the

dip is where inductive reactance is influencing the SWR value. The “dip” itself shows where the values of

these two reactances come closest to cancelling one another out, giving us an SWR of about 1.4.

Ideally our antenna would give us a very low SWR over a broad span of the frequencies we hope

to work on a particular band. In the example above, the “effective bandwidth” for this antenna would

likely be described as running from about 28.25 to 29.13 MHz, since most modern transceivers can

operate OK with SWRs below 2:1. Above this level, you really need to employ an “antenna tuner” to

modify the existing SWR as presented to the transceiver in the circuit.

We can measure SWR using our antenna analyzer or an SWR meter inserted into the circuitry inside

the “shack”. Many transceivers also have an SWR meter built-in, visible as a display on the unit’s screen.

CAUTION: SWR of an antenna, by itself, is not an absolutely reliable reflection of how “good” your

antenna is. You can have an antenna that “measures” as having a very low SWR but is still can be quietly

inefficient at using all of the power your transceiver is putting out or in using all of the incoming energy

from a received signal. The SWR value by itself should be regarded as an indicator, with the actual

performance of your antenna being a better gauge of how successful you have been.

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ANTENNA TUNER: An antenna tuner is a “device” either built onto your transceiver, or added to your

circuitry between the transceiver and the antenna. A tuner can be installed in the circuit either inside your

shack” between the transceiver and the feed line running out of your shack OR a tuner can be “remoted”

outside your shack as close to the antenna itself as practical. The tuner contains capacitors and

inductors which can either be adjusted by you (using variable capacitors and inductors, in manual tuners)

or perhaps using banks of small capacitors/inductors which can be brought into play (in so-called automatic

tuners) by electrical relays. A tuner intercepts a reflected RF signal coming back towards the transceiver

from the antenna, “blocks” it, and sends back towards the antenna a measure of the reflected signal

before this undesired RF can get to the transceiver. If the transceiver were to receive this undesired RF

feedback, it would either automatically cut back output power or face the risk of suffering damage. In effect,

the “tuner” has protected or “fooled” the transceiver into thinking everything is OK so the transceiver will

continue to put out power. The energy the tuner has “bounced back” heads back out to the antenna via the

feed line and is dissipated as heat along the way. It represents “lost” power from the entire system and

your antenna/transceiver system is not making full use of the RF signal it is trying to send out. The better

your antenna & feed line system is by itself, the less your tuner will have to do. Selecting & using tuners

is another topic to defer until a later date).

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Useful Videos On Basic Antennas: there are many websites & online videos which can provide

explanations of antennas & of the factors described above. Here are a few dealing with basic HF

antennas which are clear and well illustrated.

Antennas for Difficult Situations: video #7 25 min

https://www.youtube.com/watch?v=iKC9zv8XuPs

Making a 40m Dipole: video #86 24 min.

https://www.youtube.com/watch?v=gfLUrVnnQUI

Info on Antennas on MARC Website:

see several Ppt presentations on antennas in Archive of Training Presentations;