Graham seemed to like my Short Dipole Calculator project, calulating loading coil and antenna wire dimensions. And since that project also was posted on another forum, I have received several reports on email on how the antenna behaves on different bands.
But I have been thinking...
A mobile whip antenna can be calculated almost in the same way, by first finding the whip reactance for a given length and antenna diameter combination. Since mobile whips are very short when compared with the bands wavelength, the reactance will be capacitive. That reactance can be cancelled by inserting an incuctive reactance (a coil) at the feedpont, assuming that the antenna is endfed.
The antenna efficiency will be much increased, by moving that coil to the exact midpoint of the antenna. And in this case the needed coil reactance will be only half of the endfed version.
The coil values can be calulated using the same algorithm as found in the Short Dipole calulator.
But since such a short antenna always have a very low feedpoint impedance, we'll need some kind of matching at the feedpoint, usually a Pi-Network.
I have consulted several antenna handbooks on this subject, and also searched the internet, trying to find a good formula for calulating the values of such a network. But have found that the different formulas found give different answers. This is rather confusing, at least to me.
I know that many of the forum users are radioamateurs, so I wonder if anyone can tell me where to find the proper formulas for calulating such matching networks.
73's de LA2PJ
(Antenna freak)
PS.
For projects like this, nothing beats CB!
Hi Egil
Did you ever try to build any type of fractal antenna like Minkowski?
Hi Aurel,
I have never done any work with fractal antennas. They are best suited for UHF/SHF use, and I mostly have been experimenting on shortwave frequencies.
Regards,
Egil
Hi Guys,
Sorry to break into this thread, but could I ask a simple question?
Egil, you said for this project nothing beats CB.
Do you do most of your programming in CB?, why is CB better for you this time?
I don't have CB, just EB and IWB.
Thanks,
Andy.
P.S.
If you can describe to me in a little more detail / keywords for your formula, I will search the Internet as well, maybe another pair of eyes might help.
Thanks,
Andy.
Hi Andy,
I started out with EB, but very soon after that, I also tried CB.
It has been a great tool when trying out all kinds of weird ideas. And for small utilities, containing only a few subroutines (as in all my antenna calculators), it usually takes me longer to make illistrations than to make the program... CB is the ideal tool for such programs. Paul once told me it was an "entry level package", but it is a full fledged 32 bits Windows programming language, and though it is an interpreter, exe files are easily made, whith an overhead of 350kB (approximately).
Those graphs I suggested for you some weeks ago, was first developed in CB several years ago, when I needed them for visualisation of some key data collected with a data logger at a friends milk farm.
It is so easy (and fast) to test out new ideas with CB, that I use it for almost everything I want to try.
And finally almost all subs developed with CB, can be converted into IWB code, by just adding an ENDSUB to it. I do it all the time, and have not yet ran into trouble. I realize that not everyone will feel comfortable working this way, but for me it been just great. Have saved me a lot of time.
If you want to try it, there is a free "community version" for download here on the forum. It was posted by Graham (GWS) a while ago. He has also posted some very nice tutorials and a large amount of example code for CB.
There is also a pdf describing all CB internal constants somewhere on the forum (i beleive it was posted by Doc). And over at the CodingMonkeys forum you can find the IB archive. All IB-Standard code can be used directly in CB by changing the file extension IBA to CBA. There are tons of great examples in there.
There is really no short answer to your question, and it is diffucult to explain what I mean, but hope I managed to get "my" message through.
Egil