VHF / UHF Antennas and
Coaxial Cable – What is your
ERP ???
KARS Net Discussion – Tuesday, 23 August 2011
Presented by:
Del Partridge, W5QQ
Introduction


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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.
Goals and Objectives
Goal #1
1.
Learn some basics about antenna gain.
Goal #2
1.
Learn a little about the effect that different
types of coaxial cable have on overall antenna
performance.
Objective:
1.
To become more familiar coaxial cable types
and their effect on overall station performance.
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.
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.
Topic One
How coaxial cable effects antenna
performance.
1.
2.
3.
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.
Topic One
As an example suppose the following:



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You are using 100’ of Tandy RG-58 coax.
Your antenna is a quarter wave vertical with
unity gain. The antenna SWR is 1:1.
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.
Topic One
Coaxial Cable Loss per 100' at 2 meters and 70 centimeters
Output in watts
Output in watts
Coaxial
Charateristic Loss in db
W/50 watts
Loss in db
W/35 watts
Cable Type Impedance (Zo) at 2 Mtrs
input power
at 70 Cm
input power
RG-58U
50
5.03
15.72
9.62
3.89
RG-8X
50
4.23
18.88
7.99
5.56
RG-213
50
2.61
27.41
4.97
11.13
Bury Flex
50
1.74
33.47
3.11
17.12
LMR-400
50
1.50
35.42
2.68
18.09
LMR-600
50
0.95
40.16
1.71
23.59
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.
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.
Topic Two
.
Effective Radiated Power - unity vs 3db & 6db antennas
Coaxial
Charateristic Loss in db
Cable Type Impedance (Zo) at 2 Mtrs
RG-58U
50
5.03
RG-8X
50
4.23
RG-213
50
2.61
Bury Flex
50
1.74
LMR-400
50
1.50
LMR-600
50
0.95
ERP
Unity Gain
Vertical
15.72
18.88
27.41
33.47
35.42
40.16
ERP
3dbd
J-Pole
31.44
37.76
42.82
66.94
70.84
80.32
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.
Loss in db
at 70 cm
9.62
7.99
4.97
3.11
2.68
1.71
ERP
Unity Gain
Vertical
3.89
5.56
11.13
17.12
18.09
23.59
ERP
6dbd
J-Pole
15.56
22.24
44.52
68.48
72.36
102.36
Topic Three
The importance of knowing what
you are using between the
radio and the antenna.

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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.
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.

Where to Get More Information
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This site will compute coax loss and ERP for you:
http://m0ukd.com/Calculators/ERP_Calculator/index.php
http://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 used:
http://www.davisrf.com/
http://www.thewireman.com/
http://www.radioworks.com/hpmain.html
Where to read about Isotropic antennas:
http://en.wikipedia.org/wiki/Isotropic_radiator
http://www.smeter.net/daily-facts/9/fact15.php
http://www.qsl.net/ve3mcf/elecraft_reflect/Antenna_Measurements.t
xt
The effect of SWR on power loss:
http://www.wcerc.org/swrlosschart.asp
The 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.