base loaded whip

[biff44]

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.

 

[logbook]

Check my maths, but doesn't 315MHz give 95 cm full wave, 24 cm quarter wave? I will have to check my books to see if that is a reasonable reduction down to <6.4 cm for a helical antenna.

The first thing that came to my mind was to use a top loaded monopole rather than using a base loading coil. More later ...

 

[logbook]

According Kraus (Antennas, 3rd ed) the normal mode helical antenna has a radiation resistance of 0.6 ohms at resonance. This is probably causing your problem. Kraus suggests grounding the antenna to the ground plane and feeding in a bit further away to give a good match to the feedline. A reduction of height of x4 from a standard quarter wave monopole is very similar to the example given in the book.

 

[Higgler]

If you trimmed it to get 250 Mhz, why don't you keep trimming it so it resonates higher, say 315 Mhz. i.e. cut a winding or two off, or short some of the windings together with plastic tie wraps if you don't want to cut initially.

kch

 

[biff44]

Yes, I could cut the whip shorter, but then it would be less efficient. I want the maximum efficiency possible in the 2.5" length, which means as much metal as possible. It just does not want to tune UP in frequency.

 

[Higgler]

Antenna efficiency (and bandwidth) is based on the antenna volume used by the wires, not based on the length of the wire. With two similar antennas and a 2:1 wire length difference, if you tune both and the wire takes up the same volume, you'll get about the same gain.

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

 

[biff44]

Interesting. By "volume" do you mean just the cross section of the wire times length, or do you mean if the wire is coiled, the diameter of the coil times coil length?

 

[Higgler]

Antenna Volume = diameter of the coil times the length volume. Implies a fairly lossless wire. Resistors don't radiate very well.
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

 

[fmradio]

The intrinsic directivity of an infinitesimally short dipole is over 91% that of a 1/2-wave dipole (so says Kraus).

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).

 

[Higgler]

You can match any short antenna (at one frequency) to remove the parts that don't radiate, and as fmradio states, you end up with very low radiation resistance.

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

 

[DaveAA1A]

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