VHF / UHF Antennas and Coaxial Cable

1
VHF / UHF Antennas and Coaxial Cable What is
your ERP ???
KARS Net Discussion Tuesday, 23 August 2011

• Presented by
• Del Partridge, W5QQ

2
Introduction

• How coaxial cable effects overall station
  performance.
• Some different types of coaxial cable and how
  they effect power output at 2 meters and 70
  centimeters.
• The importance of knowing what you are using
  between the antenna and the radio.

3
Goals and Objectives

• Goal 1
• Learn some basics about antenna gain.
• Goal 2
• Learn a little about the effect that different
  types of coaxial cable have on overall antenna
  performance.
• Objective
• To become more familiar coaxial cable types and
  their effect on overall station performance.

4
Vocabulary

• dbd Used as a reference to a theoretical one
  half wavelength dipole in free space.
• dbi Used as a reference to a theoretical
  isotropic antenna in free space.
• Isotropic antenna A theoretical antenna that
  radiates the same level of energy in all
  directions when power is applied to the antenna.
  (Most often used antenna reference. Be careful.)
• ERP Effective Radiated Power (ERP) is the term
  used to describe the calculated power emitted
  from the antenna.

5
Vocabulary

• The most often used antenna reference to describe
  antenna gain is dbi. This can be misleading.
• A theoretical isotropic dipole in free space
  would have a gain of 2.15db over a theoretical
  dipole antenna with all other parameters equal.
  This reference, dbi, would make an antenna appear
  to have greater gain than an antenna referenced
  to dbd as a measure of gain.

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Topic One

• How coaxial cable effects antenna performance.
• In this example we will look at different types
  of coaxial cable and coaxial cable loss.
• We will then examine the Effective Radiated Power
  (ERP) from a unity gain antenna using different
  types of coax.
• Because coaxial cable loss goes up as frequency
  goes up we will show examples of the phenomena at
  both 2 meters and 70 centimeters.
  

7
Topic One

• As an example suppose the following
• You are using 100 of Tandy RG-58 coax.
• Your antenna is a quarter wave vertical with
  unity gain. The antenna SWR is 11.
• Your radio puts out 50 watts on 2 meters and 35
  watts on 70 Centimeters.
• Your loss at 146 MHz is 5.026 db and your power
  out is 15.716 watts.
• Your loss at 446 Mhz is 9.621 db and your power
  out is 5.546 watts.

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Topic One

•  

Note For this example a dual band FM transceiver
with 50W output on two meters and 35 W on seventy
centimeters is assumed. The antenna has unity
gain. In this example there are no losses except
as shown in the chart above.
9
Topic Two

• What would happen to the Effective Radiated power
  if we connected a dual band J-Pole.
• For the sake of discussion we will agree that the
  J-Pole has a gain of 3dbd at 2 meters and a gain
  of 6dbd at 70 centimeters.

10
Topic Two

• .

Note For example a dual band FM transceiver with
50W output on two meters and 35 W on 70
centimeters is assumed together with and assumed
antenna gain on two meters of 3db and an antenna
gain on seventy centimeters of 6db. In this
example there are no losses except as shown in
the chart above.
11
Topic Three

• The importance of knowing what you are using
  between the radio and the antenna.
• For short runs use RG 58 or RG 8X. RG-8X has less
  loss and a foam dielectric. Good and bad.
• Up to thirty or forty feet for close in work RG
  8X works.
• For longer runs look to higher quality coaxial
  cables.
• The most expensive accessory in your station can
  easily be cheap coaxial cable.

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Summary

• We have discussed two antenna measurement
  references dbd and dbi.
• We have looked at the effect of different types
  of coax on ERP.
• We have discussed the importance of knowing what
  you are putting between the radio and the
  antenna.

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Where to Get More Information

• This site will compute coax loss and ERP for you
  http//m0ukd.com/Calculators/ERP_Calculator/index
  .phphttp//www.hamuniverse.com/coaxdata.html
• The ARRL Amateur Radio Handbook and the ARRL
  Antenna Book are invaluable sources of
  information.Some suppliers on the Internet that
  I have usedhttp//www.davisrf.com/
  http//www.thewireman.com/ http//www.radioworks.
  com/hpmain.html
• Where to read about Isotropic antennashttp//en.
  wikipedia.org/wiki/Isotropic_radiatorhttp//www.s
  meter.net/daily-facts/9/fact15.phphttp//www.qsl.
  net/ve3mcf/elecraft_reflect/Antenna_Measurements.t
  xt
• The effect of SWR on power loss
  http//www.wcerc.org/swrlosschart.aspThe usual
  disclaimers apply. I received no remuneration of
  any kind from any of these suppliers. I own no
  part of any of these companies and I offer their
  names only as sources that I have purchased from
  before.