me 38 compact
I want to design a compact antenna in the frequency range from 85M to 110MHz, can anyone suggest a suitable compact solution for this?
Thanks in advance,
Ahmad,
As you kn0w, at 100MHz the λ≈3m ..
A 1/4λ antenna - whip - will have ≈750mm ..
Depnding on how compact antenna do you need, you can try the following options:
- short whip
- printed circuit whip
- spiral
- helical
and so on ..
Here you will find some examples of compact antennas - although designed for different frequencies, but, mortheless the same priciples will appply to 100MHz ..
http://www.rfm.com/corp/appdata/antenna.pdf
Regards,
IanP
Maybe the attached file could help as a reference.
Okay, thank you IanP, But i have another little question, What about input impedance of the antenna to consider when i make matching to the LNA?
These charts will provide you with information about bandwith, gain,
radiation pattern and impedance of different types of antenna's.
Thank you on1aag for charts,
But however, I can't find in them information about Zin of short whip..don't you have info about it?!
Also, if you obtained these charts from specific book, please tell me about it, and upload if you have electronic copy of it
Thanks in advance.
Ahmad,
Hi Ahmad,
There's no such thing as a good short antenna. When an antenna is too
short the impedance rises and the connected cable will mismatch the
antenna so severely that there will be hardly anything left from the
signal. You don't need an amplifier, a suitable unity gain buffer will
do the trick, and I found one. They're generally used for shortwave
reception but this one has a bandwith of 220 MHz and can be used for
your purpose. The design is actually very straight forward and will
not need more explanation, except for a few details. It's a single
sided pcb and all components should be mounted on the component side,
except T1,T2,the bnc connector and the unspecified connector for the
whip antenna. There were holes in the board to fit the transistors
and I cut away a part of the pcb to fit the bnc connector.
L1 and L2 should have a value of 4 mH for reception of the lower part
of the frequency spectrum and 470 μH for the reception of VHF signals.
The dc resistance of these inductors should not exceed 10 ohms.
They also advise you to use a short whip, something between 30 and 50cm
long, they used a spoke from a bicycle wheel with a lenght of 40 cm.
Should the whip pick up any hum from the line voltage, you could cure
this by connecting the whip to the amplifier via a 10 pF capacitor.
The entire amplifier should be mounted in a shielded box.
The voltage at the emitter of T3 should be between 6 and 8 volts,
correct this with P1 and P2. For reception of shortwave signals during
the daytime, when reception is poor, Ug2s should be between 3 and 6
volts. Correct this with P1, the lowest voltage at which a weak
station still can be heard will be the best choice. For reception of
shortwave signals during the night, when the signals are the strongest,
Ug2s should be around 6 volts. Correct this with P1 when receiving a
strong signal and adjust for minimum distortion. They don't say anything
about vhf reception. I translated the most important details of the
article in case you couldn't read Dutch.
About the charts; they were scanned from photocopies that were made
from reprints from "The Electronic Engineer, januari 1968."
on1aag.
Thank you "onlaag",
I think it will be usefull, but i don't knwo howt to extract the "djvu" file !
Hi Ahmad,
This will help:http://bama.sbc.edu/utilities.htm
on1aag.
I have another question,
for the antenna seen in the next figure,
What"s its input impedance?
Gain?
Polarizaion type?
obtained from here:
Thanks alot
Ahmad,