How to increase the electrical length in this antenna design

[ghb]

antenna thoughts plz

30MHz - 1GHz

fixed length of 1M

max diameter of 5.08 CM

no ground plane to speak of

radiation pattern is not important only impedance bandwidth.

matching unit is possible but cannot be elaborate.

i've been looking at this for a couple weeks off and on. to increase electrical length i've envisioned a normal (broadside) mode helix, which via simulations looked promising, however this was done on infinite gp.

the results after building one were not as promising.

does anyone have thoughts on how to increase the electrical length? or an antenna style that maybe suitable. i would love to discuss this.

thanks,

ghb

 

[flatulent]

Re: antenna thoughts plz

You can put several dipoles of different lengths in parallel. Putting an attenuator on the input will keep the return loss low.

One of the dipoles could be a conic one which acts like a high pass filter.

 

[biff44]

Re: antenna thoughts plz

A meter is a long length. Seems like if you had a travelling wave structure, with a large number of non-resonant radiating structures spaced along the length, you would get the broadband impedance match. Maybe small loops? Theory of small reflections says that with the right coupling taper, you should have little reflection.

 

[throwaway18]

Re: antenna thoughts plz

>30MHz - 1GHz
Any antenna with such a wide coverage is going to be a tradeoff.

The white stick antennas sold for use with scanning receivers are usualy just a coil of wire in a plastic tube.

What sort of feeder?

If you feed it with coax then Unless you use a balun or choke of
some sort you will find that the outside of the coax braid forms
a part of the antenna. The length and orientation of the coax
will have a big effect on the impedance and radiation pattern.

>radiation pattern is not important only impedance bandwidth.

How about efficiency?
A fifty ohm resistor and a meter of wire will be somewhere in the
ballpark over the whole range.

>which via simulations looked promising, however this was done on infinite gp

A simulation of an antenna with a big ground plane tells you nothing
about how an antenna with no groundplane will perform.

>You can put several dipoles of different lengths in parallel
My feeling is that 5cm diameter is too close for that to work well.

>A meter is a long length.
At 30MHz with no ground plane it is a short length.

>a large number of non-resonant radiating structures

A quarter of a wavelength at 1GHz is 75mm so you need your "radiating
structures" to be smaller than that. At 30MHz it will be useless.

>Theory of small reflections
Hmm, can't seem to find a good explanation of that with a two minute google

>says that with the right coupling taper, you should have little reflection.

Hmm, I suspect that doing that within the specified dimentions would be
impractical.

 

[biff44]

Re: antenna thoughts plz

Throwaway, you did not look very hard. I got 5 pages of hits on "theory of small reflections"--a theory that every microwave engineer should have been taught somewhere back around Sophomore year!

Here is one:
www.paper.edu.cn/scholar/download.jsp?file=wangjunhong-6

The cable "slots" are really circulating currents that should not be resonant over this frequency range, so the coupling should be similar at 30 MHz and 3 GHz.

Also this one starting around page 20:
http://cc.ee.ntu.edu.tw/~thc/course_mckt/note/note4.pdf

 

[throwaway18]

Re: antenna thoughts plz

A long mathematical formula with no words is not what I consider
to be a good explanation.

Below 1GHZ is not microwave.

The leaky feeder paper you linked has a graph showing coupling
loss between 65dB and 85dB. That's totally useless for a
general purpose antenna, the proverbial bit of wet string
works better than that.

>The cable "slots" are really circulating currents

How can you get a circulating current at 30MHz when the
whole thing is smaller than a quarter of a wavelength?

Perhaps you could draw us a diagram showing the mechanical
arrangement you are thinking of?

> coupling should be similar at 30 MHz and 3 GHz.

My understanding is that even if you manage to couple a 30MHz signal to a 50mm radiating element
it will radiate/receive very little signal. When transmitting somthing like 99.9% of the power will be
lost as heat in the element instead of radiated. On receive it just won't pick up much of the energy passing by.