30 MHz Broadband Antenna

[cbsirio]

Hi all,
A friend of mine is having a problem designing a broadband 30MHz antenna.
He has a wire antenna (about 30 cm) and he is designing a matching network, but he is always having a narrowband design.

Can you tell me if there are techniques to make a short wire antenna (intrinsically narrowband) Broadband?

Thank you very much in advance!

Sirio

 

[flatulent]

You are up against the laws of physics. Even though you may be able to make a matching network that works, the losses will be great. The radiation resistance compared to the component resistances will cause these losses.

 

[biff44]

If it were a low power application, is there someway to get a broad band, although inefficient, match that would not have any sharp resonances?

 

[throwaway18]

The feeder and the ground is going to make a big difference to the impedance and radiation pattern. If you fed it with coax the outside of the coax will form part of the antenna eg ten feet of coax is a quarter wave at about 28MHz

It may be better to use a metal tube instead of a wire.

There are mobile antennas for 27MHz that are about 40cm long and give a reasonable 50ohm match when used with a large groundplane. They consist of a few hundred turn s of wire around a fibreglass rod.

You could try a big coil and a 6dB resistive attenuator (pad).

 

[cbsirio]

Well, no way to break law of phisics!

But the need is to design a matching network (with high power = 600W) to have a broadband match (about 5 MHz @ 30MHz), no matter about the the efficiency of the antenna overall...

How can I realize a matching network with this characteristics? No resistors and so high bandwidth?

Can you help me please?

Thank you all!

Sirio.

 

[throwaway18]

600Watt at 30MHz into a 30cm long antenna?

You will need thick wire in the coils in your matching inductors to stop them melting.
You are likley to have problems with electric arc's between the antenna and ground.
Stand far away when you turn it on.

Why do you want to do this?

 

[flatulent]

You start with the input impedance measured or calculated at several points across the frequency range. The match will be exact at these points and not inbetween. Therefore the greater the points, the smaller the error between them.

Then you need an LC network with the number of elements equal to or greater than the number of frequency points.

Then you derive the input impedance (a polynomial in S) of the network as connected to the load. This will have coefficients related to the element values stated in algebraic form, such as L1, L2, C1, and C2.

This will give you a set of polynomials at each frequency where you substitute the load impedance value at the frequencies.

You then solve this set of N simultaneous equations for the numeric values of the LC values.

With your power level the components will be very large to handle the hundreds of Amperes of current and many kV. They may be so large that they have a total volume greater than a m^3 especially when you have to orient the inductors to not have any mutual inductance.

Nuhertz makes a Zmatch program that does the calculations. It has a 20 day free trial period. If you are on a commercial project you can buy the program.

Another alternative for your antenna would be a several turn loop that is 1 m in diameter. This form of antenna was commercially sold some years ago for the amateur service. It had a manually adjusted capacitor. It is no longer sold because it had such low efficiency and the amateurs stopped buying it.

One further item is that the antenna impedance will be a small resistance in series with a very large and variable capacitive reactance. If you can make the antenna have a large diameter the reactive part will be smaller and less variable.

Another item to check for is the stability of your power amplifier working into the load. Even though it may work at the design frequency, the out of band reactance of the load may cause oscillations.

 

[flatulent]

I have just thought of the following. If you can do not use straight wire. Take 3 m of wire and wind it into a helix. Make the diameter as large as possible. This will produce a larger real part and less reactive part of the input impedance.

The ground plane (box top) is part of the antenna and will affect the impedance.

You can make a vertical dipole by making two of the above and feeding the center with parallel wire transmission line.

Do some experimenting and use an impedance bridge to take measurements.