4.433MHz ~10m range telescopic antenna design? - Page 1
Forum section: electronics / rf-microwave

author: strawberry
author_rank: Super Contributor
timestamp: July 09, 2023, 11:07:20 am
content: resonant load A class PA with some LP filterhow to find antenna loading impedance?how to design filter?

author: RFDx
author_rank: Regular Contributor
timestamp: July 11, 2023, 05:56:52 pm
content: Quote from: strawberry on July 09, 2023, 11:07:20 am

author: radiolistener
author_rank: Super Contributor
timestamp: July 11, 2023, 07:19:16 pm
content: Quote from: strawberry on July 09, 2023, 11:07:20 amif you have antenna you can measure it. Otherwise you can use antenna simulator to find it's impedance.Quote from: strawberry on July 09, 2023, 11:07:20 amJust get some filter design tool, enter impedance and cut-off frequency, filter type, filter order and get caclulated result with frequency response.

author: strawberry
author_rank: Super Contributor
timestamp: July 12, 2023, 05:06:08 pm
content: B..C class ampinitial test with wire antenna(2m long) works well but not efficient(such short antenna apparently is less than 1ohm capacitative load)18uH loop antenna( hopefully will have directional properties with higher efficiency and smaller size) magnetic coupling with VEF202... internal SW coils

author: iMo
author_rank: Super Contributor
timestamp: July 12, 2023, 07:01:27 pm
content: You have to "match" the impedance of all components in your design string.PA_output matches the filter input matches filter output matches antenna_feed.10k load at your filter output is rather unrealistic as there is not such impedance with typical antennas (perhaps a half wave wire would be around 5k).You may match 2m long wire at 4.3MHz, sure, but the efficiency will be low (a 35m long wire would be half wave, 2m long wire will have perhaps 5% efficiency when properly matched).A magnetic loop may work too, with similar efficiency when with around 1m diameter.PS: For example, I would estimate the capacitance of the 2m wire as, say, C_ant=50pF, and you would need an inductor at the feed point of the antenna (in series with the antenna wire) with an L_ant=26uH such it resonates at your 4.433MHz.

author: iMo
author_rank: Super Contributor
timestamp: July 12, 2023, 08:12:09 pm
content: Here is a 4NEC sim with 2m long wire vertical, f=4.433MHz, 46uH in the base of the antenna, matched to 450ohm (ie. 1:9 trifilar unun with 50ohm TX filter output).

author: Wallace Gasiewicz
author_rank: Super Contributor
timestamp: July 14, 2023, 08:22:06 pm
content: The radiation resistance of such a short antenna is VERY small and iMO is right about the efficiency.You can get a Magnetic Loop to be somewhat efficient at these frequencies but as mentioned the diameter of the loop would be more than one meter for your lower frequency requirement..The Surface Area of the loop is a big factor in radiation resistance and thus efficiency, the bigger the tubing the better.In transmitting mag loops even soldering is not considered adequate, usually silver solder or welding for connections to eliminate as much ohmic resistance as possible.Also the power is restricted , the loop will generate tens of KV and a really good capacitor is needed for tuning.A Mag Loop is really just a physically large LC circuit.