Ramsey Electronics DA25 User Manual

BROADBAND

DISCONE ANTENNA

KIT

Ramsey Electronics Model No. DA25

Looking for a broadband antenna with a full 360 degree
coverage? Discover what communication professionals have
known for years using a “discone” antenna. Use this antenna to
bring a multitude of signals out of the noise making it ideal for
scanners and Ultra High through Microwave Frequency
receivers! Search the airwaves for signals with this unique kit!

• Omni directional performance, no need to point in any direction!

• Learn about antenna theory, and what makes the discone an ideal

broadband antenna!

• Covers all frequencies between 450 MHz and 2500 MHz, and you’ll

learn why!

• E-Z cable connection, industry standard BNC type connector.

• Outperforms models costing tens to hundreds of dollars more.

• Super small in size for easy mounting almost anywhere! An ideal

“apartment” size antenna!

• All hardware and pre-drilled metal work included.

• “Forgiving” design gives you a high performance antenna each and

every time.

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RAMSEY TRANSMITTER KITS

• FM100B Professional FM Stereo Transmitter

• FM25B Synthesized Stereo FM Transmitter

• MR6 Model Rocket Tracking Transmitter

• TV6 Television Transmitter

RAMSEY RECEIVER KITS

• FR1 FM Broadcast Receiver

• AR1 Aircraft Band Receiver

• SR1 Short wave Receiver

• SC1 Short wave Converter

RAMSEY HOBBY KITS

• SG7 Personal Speed Radar

• SS70A Speech Scrambler

• BS1 “Bullshooter” Digital Voice Storage Unit

• AVS10 Automatic Sequential Video Switcher

• WCT20 Cable Wizard Cable Tracer

• ECG1 Electrocardiogram Heart Monitor

• LC1 Inductance-Capacitance Meter

RAMSEY AMATEUR RADIO KITS

• DDF1 Doppler Direction Finder

• HR Series HF All Mode Receivers

• QRP Series HF CW Transmitters

• CW7 CW Keyer

• CPO3 Code Practice Oscillator

• QRP Power Amplifiers

RAMSEY MINI-KITS
Many other kits are available for hobby, school, Scouts and just plain FUN. New
kits are always under development. Write or call for our free Ramsey catalog.

DA25 KIT INSTRUCTION MANUAL

Ramsey Electronics publication No. MDA25 Rev 1.3

First printing: November 2001

COPYRIGHT 2001 by Ramsey Electronics, Inc. 590 Fishers Station Drive, Victor, New York

14564. All rights reserved. No portion of this publication may be copied or duplicated without the
written permission of Ramsey Electronics, Inc. Printed in the United States of America.

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Ramsey Publication No. MDA25

Price $5.00

KIT ASSEMBLY

AND INSTRUCTION MANUAL FOR

BROADBAND DISCONE

ANTENNA KIT

DA25

TABLE OF CONTENTS

Introduction .................................4

Discone Circuit description .........8

Parts list ......................................10

Assembly instructions .................11

Installation and Important notes .. 16

Using your DA25 ........................17

Troubleshooting guide ................17

Warranty ..................................... 19

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RAMSEY ELECTRONICS, INC.

590 Fishers Station Drive

Victor, New York 14564

Phone [phone]

Fax [phone]

www.ramseykits.com

INTRODUCTION

In today’s ever growing “wireless” society, it almost seems a bit ironic that
antennas have become less and less the topic of interest in hobbyist circles.
The recent advances in wireless technology have shrunk antennas to ever
smaller an unobtrusive sizes. An example of this is the cable television industry.
They have removed the larger “traditional” antenna arrays that were once
commonplace for TV reception and replaced them with a single wire or two
entering the household. Advances in the semiconductor industry have provided
engineers with the tools to pull the smallest signals from the airwaves with
better noise performance than could have been dreamed of when the
technology of radio reception was envisioned. Advances in satellite technology
have reduced the size of a reception “dish” from over 12 feet in diameter to a 1
foot round platform!

Antenna design certainly has not made the “quantum leap” that was brought on
with the advances in the semiconductor industry, but it is just as important as it
was in those early days of radio. The original aerials, or reception antennas,
had to provide enough signal to overcome the ever present noise and allow the
early receivers to detect and demodulate signals. These early antennas were
quite large (we’ll talk a little more about this later) due to the lower frequencies
being transmitted. Again, more recent improvements have allowed us to use
higher frequencies with significantly smaller antennas.

With less and less demand for consumer antennas, the market price of these
commodities has increased. As many of us have discovered, even the lowest
cost antennas run in excess of one hundred dollars! While they are necessary if
we intend to use the antenna commercially or for television reception, it simply
is too much for a hobbyist to invest for use with a monitoring receiver. Enter the
Ramsey line of discone antennas, allowing us to “tinker” with the airwaves at an
affordable price.

Ramsey Antennas 101:

Before we break open our discone kit, lets talk about what makes an antenna
tick, and some of the terms used to define antenna performance.

How Fast are Radio Waves?

If one were to “whip” the end of a taught length of rope, you could observe the
wave created traveling down the rope to it’s end. Going back to our physics
class, recall that the speed of any object is the distance it travels divided by the
time it takes to get there, or Velocity = Distance / Time. The time a wave takes
to travel is dependant on the type of wave and the transmission medium. The
wave in our rope example can take seconds to traverse down the length of the
medium. Sound waves travel about 1100 feet every second; if we called out

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before we snapped the rope, the sound waves would arrive much quicker than
the “rope wave” would. In the case of radio waves, the rate at which the waves
travel is much faster, reaching the speed of light (186,000 miles / second, or
about 3x10

8

meters / second) in a vacuum. Radio waves do travel slightly

slower in air however. In a wire transmission line, they travel even slowly!

Frequency and Wavelength

Since all antennas collect electromagnetic waves, lets take a moment to think
about the wave motion of the radio wave itself. Try to picture a repeating
sinusoidal waveform moving down a line (oscillating). A wave that repeats itself
has a certain period (amount of time) that it takes to complete a full cycle.
Since this cycle is regular, we say that the wave has a frequency of repetition.
This frequency in fact is the reciprocal of the time it takes for the wave to
complete one full cycle, mathematically speaking f = 1 / T. By the same token
the time and frequency are related by the expression T = 1 / f.

The distance in free space that the wave takes to repeat itself is said to be the
wavelength and can be calculated using the same velocity equation. By
rearranging the velocity equation algebraically, we can say that the
Distance = Velocity x Time. Since we will approximate the velocity to be the
speed of light (“c”), once the Time is determined we can solve for the distance
traveled which is the wavelength; usually denoted as the Greek letter lambda
(“λ”) reducing our equation to λ = v x T. In English, the wavelength is equal to
the velocity multiplied by the period of the waveform. Pretty neat, huh!

Electromagnetic Wave

One Wavelength

One Cycle

Some Amount of Time

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What Are We Driving At?

Time to pull some of that theory together and get some answers:

Since: λ = v x T

And T = 1 / f

We can substitute and get:

λ = v / f

Since the velocity equals “c” we wind up with:

λ = c / f

The wavelength of the radio wave equals the speed of light divided by the
frequency.

Lets plug some numbers into our equation and work out a few wavelengths. We
should notice some other properties of electromagnetic waves.

If f = 450 MHz (the wave cycles 450 million times in a second) then λ = 3x 10
450 x 10

If f = 2500 MHz (the wave cycles 2500 million times in a second) then λ = 3x
10

6

or .666 meters for a full wavelength.

8

/ 2500 x 106 or .120 meters for a full wavelength.

8

It’s important to note that as the frequency of a wave increases, its wavelength
decreases. Keeping in mind the introduction section where we talked about
antenna size, lets consider the “old” days of radio. The common use of low
frequencies meant much longer wavelengths and significantly larger antennas
for reception. Today's modern electronic devices tend to operate at much
higher frequencies and thereby require smaller antennas to operate properly.

Determining the Resonant Frequency of the Antenna

Let’s explore another factor in antenna as well as radio design, the resonant
frequency of the circuit. Recalling that we would like our discone antenna to
work over a large range of frequencies, we need the antenna system to be
optimized for the full desired range. Resonance in an antenna circuit occurs
when the antenna length exactly matches the wavelength of the desired
frequency. To make an antenna resonant over a range of frequencies, it needs
to look like a multitude of lengths.

Looking at the desired waveform, the shortest length of wire that will resonate
at a given frequency is one which is just long enough to permit an electric
charge to travel from one end to the other and then back again in the time of

/

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