Презентация Low-Band Receive Antennas онлайн
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- Тип файла:ppt / pptx (powerpoint)
- Всего слайдов:98 слайдов
- Для класса:1,2,3,4,5,6,7,8,9,10,11
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Слайды и текст к этой презентации:
№4 слайд
![Why do we need separate TX](/documents_6/ad5c2e80241aa430e06e17ac1b0f9b76/img3.jpg)
Содержание слайда: Why do we need separate TX and RX antennas?
Because, they have different requirements:
TX antennas need to deliver strongest possible signal into target area compared to other antennas.
Efficiency and gain are most important factors.
RX antennas need to have best Signal to Noise Ratio (SNR) – gain and efficiency are not necessary.
№8 слайд
![NO nothing has been gained!](/documents_6/ad5c2e80241aa430e06e17ac1b0f9b76/img7.jpg)
Содержание слайда: NO – nothing has been gained!
The pattern is still practically identical
Front/Back is the same
Front/Side is within 0.47dB
Unwanted noise is external to the antenna. Because the directivity of the two antenna systems is the same, the Signal/Noise ratio is exactly the same for both.
We must use Directivity when comparing RX Antennas, not gain.
№9 слайд
![How much Negative Gain can we](/documents_6/ad5c2e80241aa430e06e17ac1b0f9b76/img8.jpg)
Содержание слайда: How much Negative Gain can we tolerate with RX antennas?
Modern receivers are very sensitive.
If you can easily hear an increase in background noise when switching from a dummy load to an RX antenna under quietest conditions, then gain is sufficient.
Minus10 to minus 20 dBi Gain is generally fine for most occasions.
№20 слайд
![Theory of Operation Despite](/documents_6/ad5c2e80241aa430e06e17ac1b0f9b76/img19.jpg)
Содержание слайда: Theory of Operation
Despite the shape, actually a pair of verticals
Feedline on top and bottom gives crossfire phasing towards feedpoint when elements closer than ¼ Lambda
Terminating resistor is equal to feedpoint impedance, and ensures equal current throughout
Thus, vertical elements have phase difference of 180 deg plus electrical length of connecting wires (slightly more than element spacing)
This gives the cardioid pattern
№35 слайд
![Feeding Elongated Loops](/documents_6/ad5c2e80241aa430e06e17ac1b0f9b76/img34.jpg)
Содержание слайда: Feeding Elongated Loops
Impedances range from 500 Ohms in K9AY, to 950 Ohms in Deltas and Flags.
Important characteristics:
Lowest possible capacitive coupling between primary and secondary windings.
Low loss, as signals are weak
Good SWR if you want to phase loops into an array of loops
№45 слайд
![How High? Not as critical as](/documents_6/ad5c2e80241aa430e06e17ac1b0f9b76/img44.jpg)
Содержание слайда: How High?
Not as critical as many think
General rule:
Higher Beverages produce higher output
Higher Beverages have larger side-lobes
Higher Beverages have a higher elevation angle
Higher Beverages have a wider 3-dB forward lobe
Laying on ground to 6 meters high is acceptable
1.5 x Antler Height is good idea!
2.5 meters is a good compromise
№46 слайд
![Ground Quality The better the](/documents_6/ad5c2e80241aa430e06e17ac1b0f9b76/img45.jpg)
Содержание слайда: Ground Quality
The better the ground, the lower the output
Ground quality has little impact on radiation angle
The poorer the ground, the less pronounced the nulls between the different lobes
Directivity remains almost constant
Beverage does not work well over salt water
№65 слайд
![Grounds One -foot ground rod](/documents_6/ad5c2e80241aa430e06e17ac1b0f9b76/img64.jpg)
Содержание слайда: Grounds
One 8-foot ground rod may suffice
Will probably need two or more to stabilize the ground system
Can supplement it with a number of short radials to form capacitance hat to earth
On coax end of antenna, do not ground the coax braid
Ensure the coax braid ground is no closer than 5 meters to the ground attached to the transformer
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