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Amateur Radio Antennas - For discussion of all amateur band designed antennas and related accoutrements. This includes base, handheld, mobile and repeater usage. For commercial antennas on the amateur bands please use Commercial Radio Antennas below.

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Old 03-14-2018, 5:23 AM
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Default What is this element on dipole

Hello everybody! I'm new in HAM and I'm in process of learning. I have one question. I found this schema of HF dipol antenna and I want to figure out what is purpose of component at number 2. I don't know germany and google translate doesn't help me. What does this thing do?

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https://prnt.sc/iqzwik
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Old 03-14-2018, 7:16 AM
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Those appear to be traps, and inductor and capacitor in parallel.
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Old 03-14-2018, 7:48 AM
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This is what makes me confused. On schema is inductor and resistor?
And traps consists of capacitor and inductor?
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Old 03-14-2018, 7:51 AM
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Their purpose is to widen the resonant frequency range of the antenna. Lower frequencies will pass through the inductor, and higher frequencies will pass through the capacitor.
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Old 03-14-2018, 8:21 AM
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Quote:
Originally Posted by henri_007 View Post
This is what makes me confused. On schema is inductor and resistor?
And traps consists of capacitor and inductor?
The schematic is confusing because they don't use the standard symbol for the capacitor and they have no component values indicated.

Antenna traps are tuned circuits and as such use coils and capacitors, so the devices in parallel with the coils in the schematic are capacitors.
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Old 03-14-2018, 8:40 AM
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A rectangular box isn't a standard symbol for any discrete electronic component. It's often used for ICs, but in that case, the rectangle is labeled with the chip ID, and the pin number for each connection to the IC is generally marked.

Omitting component values is also bad form.
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Old 03-14-2018, 9:53 AM
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Well, sort of. A trap is a parallel resonant circuit that will effectively end an antenna at a particular frequency to make it resonant there, where the antenna would otherwise be too long to naturally resonate at that frequency. The part of the antenna that extends beyond the trap can be tuned and used for lower frequencies than what the trap is tuned for. More traps can be used to break up the antenna to resonate at other discreet frequencies.

A typical example of a trap dipole would be breaking up an 80m dipole to resonate on 40m also by placing a parallel resonant circuit tuned to 7MHz about 33ft away from the feedpoint in the longer 80m 65ft element. The trap will also tend to shorten the overall length of the antenna on the lower frequencies, and in the case of a trap 40/80m dipole the overall length would be shortened from about 133ft to 105ft.
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Their purpose is to widen the resonant frequency range of the antenna. Lower frequencies will pass through the inductor, and higher frequencies will pass through the capacitor.
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Old 03-14-2018, 9:58 AM
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Do an internet search for trapped dipole antenna, you will find a wealth of information.

Below is a picture of a 40 meter/80 meter trapped dipole antenna. The traps are the capacitors and inductors in parallel.

In operation the traps limit (block) the 40 meter signal so it uses only the inner part of the antenna. For 80 meter operation the traps allow the 80 meter signal to use the entire antenna.

In simple technical terms the traps form parallel tuned circuits. At their resonant frequency, in this case the 40 meter band, the traps present a high impedance to the signal, limiting it to the inner part of the antenna.

There are many construction techniques for building traps, but in general, schematically they are shown as a capacitor and inductor in parallel.

Trap antennas are not limited to just two band, there multi-band antennas that can be purchased or built.
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Old 03-14-2018, 7:06 PM
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The rectangle is the standard IEC symbol for a resistor. Some traps on a dipole or a vertical trapped antenna do not have a 'physical" capacitor that you can see, but it's made from a tube over the coil to increase the self-capacitance. The resistor may be there to broaden the stop band frequencies of the trap.
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Old 03-15-2018, 6:47 AM
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Thank you for your answer!



I found https://prnt.sc/ir28cy schema and there are values for elements. Says that resistor is 300 ohm? I'm little confused with this

I have this antenna from schema and I opened this element (on number 2). There is really inductor and resistor. (3 resistors of 910 ohm connected in parallel).
Is this maybe some kind of filter?
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Old 03-15-2018, 6:53 AM
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Quote:
Originally Posted by jonwienke View Post
Their purpose is to widen the resonant frequency range of the antenna. Lower frequencies will pass through the inductor, and higher frequencies will pass through the capacitor.
I started to post that I suspected a resistor but you found that out. Resistors lower the Q of a circuit like that (broaden the effect if you will and make the value less critical for one thing). I have done that in tuned circuits. The capacitance apparently being used is just the stray capacitance.

Last edited by dlwtrunked; 03-15-2018 at 6:59 AM..
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