base loaded whip
base loaded whip
(OP)
I have a 2.5 inch space above my board for a whip type antenna. I am at 315 MHz, so that is too short for a quarterwave. I found some helical wound berylium copper, trimmed it to 2.5", and it resonated down at 250 MHz. I screwed around with base loading the antenna with a series capacitance, but was unable to tune it much higher than 273 MHz.
So, what am I doint wrong. IF the length was too short, I could electrically lengthen it witha series (base load) inductor. So why, if the length is too long, can I not do the same thing with a capacitor?
I could just trim the helical length shorter until it resonated at 315 MHz, but I assumed that the longer length, properly resonated for center frequency, would give me a more efficient antenna.
So, what am I doint wrong. IF the length was too short, I could electrically lengthen it witha series (base load) inductor. So why, if the length is too long, can I not do the same thing with a capacitor?
I could just trim the helical length shorter until it resonated at 315 MHz, but I assumed that the longer length, properly resonated for center frequency, would give me a more efficient antenna.





RE: base loaded whip
The first thing that came to my mind was to use a top loaded monopole rather than using a base loading coil. More later ...
RE: base loaded whip
RE: base loaded whip
kch
RE: base loaded whip
RE: base loaded whip
With lossless conductors, you can get good gain out of tiny antennas, but realistically, the more volume you have the better. Gain falls off fast for short antennas.
Hence, I'd suggest you try to cut it shorter, then stretch the coils to fill your alloted volume, then tune it as needed. Electrically short antennas are tricky and very narrow bandwidth.
You didn't say how much width, or radius you have. If you have ample radius, a patch antenna filling your volume might work. (you'd need 3.5 inch length x 1" width x short height).
kch
RE: base loaded whip
RE: base loaded whip
There's alot of antenna work in the area of electrically small antennas. Laws of physics kick in when you want gain and bandwidth. Technically(i.e. mathematically) you can get an efficient antenna of tiny size if your metal wires are lossless superconductors. i.e. 1/1000 Lambda antenna could be zero dBi gain at tiny tiny bandwidth.
search for Wheeler, he invented the "Wheeler cap" method to put a small antenna inside a metal box/cap and measure VSWR to find an efficiency of small antennas.
kch
RE: base loaded whip
The problem is that the radiation resistance of the short dipole or monopole is so small, and its input reactance so large that most of the available r-f power is dissipated in the losses of the tx output stage circuit, r-f ground reference (if any), and the network needed to Z-match the short antenna.
The diameter of the radiator determines its reactance, and the reactance of larger OD radiators changes slower with change in frequency (ie, has better SWR bandwidth).
RE: base loaded whip
Much interest has been generated for superconductive antenna wires so that the loss in the antenna (with low radiation resistance and high current) is minimized, and hence the gain is maximized.
I've seen standard inductive matching, (base loading, or mid loading) and transformers at the antenna, but not much in the way of superconductors (yet). If you look at short ferrite loaded antenna gain curves with and without capacitive matching, the matching can really help.
kch
RE: base loaded whip
I think you can accomplish your goal by keeping the loading inductor at a small diameter and squeeze/lengthen it ( coil ), if not adding or subtracting turns...
The effective height 'G' will remain about the same...
Also think about top loading @ 6ghz., no problem...
TNX Dave AA1A