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Author Topic: antenna impedances  (Read 962 times)
N4JTE
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Posts: 1155




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« on: June 02, 2013, 06:09:11 PM »

I would like to ask a question that might be helpful to myself and other antenna builders.
When designing/ building an antenna I use some memory retained from much reading and past history to figure out the expected impedance of the antenna before baluns or matching/ feeding networks etc, some are obvious but some are not. Lets assume all the following antenna are at a respectable height of 1/4wl or more, what will the impedance be for the following antennas?
1; Dipole
2; 135 ft flat top wire on various bands.
3; EDZ built for the frequency of choice.
4; 2 element full size horizontal beam for frequency of choice at .15wl spacing.
5; Vertical monople, ground mounted with extensive ground radials.
6; Same as number 5 with parasitic reflector at .15wl spacing.
7; Quad loop with average height at 60% on top element for band of choice.
8; 80 Horizontal loop on various bands.
9; Phased wire dipoles at 1/4wl spacing.
10; Inverted L with substantial ground radial system.
11; End fed 135 ft at design band of choice.
12; 43 ft vertical on various bands.
Tnx
N4JTE

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WX7G
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Posts: 5948




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« Reply #1 on: June 02, 2013, 07:47:23 PM »

I could tell you the answers but I think it would be good for you to find them in the ARRL Antenna Book. Another way to find out is to model each antenna using a NEC program such as EZNEC.
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K4SAV
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Posts: 1834




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« Reply #2 on: June 03, 2013, 06:15:01 AM »

All those choices either have a variable range of impedances possible depending on the exact parameters involved, or for the conditions stated have a range of impedances that may vary by as much a a couple of orders of magnitude.  I doubt you will find all these in an antenna book either.  The best way to answer these questions is to model each antenna.  You will see there is no one answer which is correct for all the different ways to build these.  Giving you a "typical" number doesn't say anything about what is possible.

The dipole you can find in an antenna book.  The answer will be somewhere between 55 and 100 ohms depending on height and ground quality, unless the wire used is very small or very large.

The 1/4 wave vertical monopole will have the smallest range of any of the choices listed, and you may be able to find it in an antenna book.  It will generally be in the range 35 to 40 ohms unless very skinny wire is used.  However most hams don't have an "extensive" radial system or ideal construction equivalent to that of a generalized model.

Jerry, K4SAV
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W5DXP
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Posts: 3551


WWW

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« Reply #3 on: June 03, 2013, 08:44:42 AM »

2; 135 ft flat top wire on various bands.

How about 130 ft? http://www.w5dxp.com/smith.htm

Complete article at: http://www.w5dxp.com/notuner.htm
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73, Cecil, www.w5dxp.com
The purpose of an antenna tuner is to increase the current through the radiation resistance at the antenna to the maximum available magnitude resulting in a radiated power of I2(RRAD) from the antenna.
WB6BYU
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Posts: 13113




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« Reply #4 on: June 03, 2013, 08:46:48 AM »

...what will the impedance be for the following antennas?


I'll comment on some of the "easy" ones, but don't forget to include the effect
of the impedance transformation on the feedline as well as any changes in the
impedance due to common mode currents.

1; Dipole
Roughly 40 to 100 ohms, depending on height above ground, etc.

2; 135 ft flat top wire on various bands.
varies widely, typically high (thousands of ohms) when the antenna is a multiple
of 1 wavelength long.


4; 2 element full size horizontal beam for frequency of choice at .15wl spacing.
depends on the tuning of the elements.  Typically adjusting for higher gain
means a lower feedpoint impedance.  Probably in the 10 - 50 ohm range.


7; Quad loop with average height at 60% on top element for band of choice.
That isn't a very clear description.  A single full wave loop is typically in the
90 to 130 ohm range, but it depends on the configuration:  a tall skinny loop fed
at the bottom can be 50 ohms or less, while fed in one side it may be 200 ohms.


8; 80 Horizontal loop on various bands.
Varies a lot, depending on the configuration and what part of the 80m band you
tune it for.  On those bands where it is a full multiple of 1 wavelength, it will typically be
around 80 to 90 ohms on 80m increasing to around 400 ohms on 10m.



10; Inverted L with substantial ground radial system.
Totally depends on the dimensions.

11; End fed 135 ft at design band of choice.
On the bands where it is a multiple of 1/2 wavelength, it will have a high
feedpoint impedance (couple thousand ohms or so), but that will depend on
what it is fed against.  On 160m it is probably in the 20 - 50 ohm range.


12; 43 ft vertical on various bands.
varies widely.  
http://www.vk1od.net/antenna/multibandunloadedvertical/index.htm

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