Question on Antenna design - will mismatch alone change antenna radiation pattern ?


ON5KQ
 

Thanks for answers. Jim and Glenn...
I am just curious, why I felt to be often unsuccesful with antennas, which are planned to be resonant on 15m (for example) usually work by far less good on a different band compared to  a well matched.

So I will try to build it during the coming summer ...

See you on Wednesday meeting...

Ulli, ON5KQ


Jim Lill
 

If the mismatched portion of the system includes the feedline, a "hot" feedline can skew the pattern in some cases.

On 7/20/21 10:37 AM, Glenn Elmore wrote:

Not claiming to be an expert but I would say that pattern is due to response in the far field.  As long as the 'matching' or other radiating mechanisms don't  have significant radiation compared to the 'antenna' then that response won't change.

Having said that, the *apparent* pattern can change. If mismatch gets high enough that other mechanisms, not intended to be included in what one calls the 'antenna', become significant compared to the desired/intended one then the pattern and performance can indeed change be different. 

This second situation is essentially what happens with common mode ingress. That which one *thinks* is the antenna is not really the total of what is providing response at the receiver.  Once intended antenna (mismatched) signals get down to the level of other signals, such as CM then the effective pattern is no longer entirely due to the intended antenna.

This problem can be aggravated by electrically small antennas which though they intercept the same aperture and, were they matched, would deliver the same signal power, deliver very small voltage or current levels due to their preamplifiers because of the low radiation resistance and high reactance and imperfect matching.

Perhaps surprisingly, were we able to perfectly match a [1 cm] dipole at 160m it would have just about the same performance as a full half wave. Of course we can't do that with materials we have, probably not even with super conductors so in practice this doesn't occur. 

But the general idea of using "something" as an antenna, no matter whether the feedpoint impedance is near something easy to access with reasonable matching materials, is perfectly fine as long as one compares the resulting mismatch to the target at a particular frequency of interest.  That target starts to get difficult at the high end of HF where we want to get increasingly close (or below) KTB. It's for this reason that either a loop, such as described by LZ1AQ, or my dipoles both fail to achieve the ITU 'quiet rural' noise floor at 10m.  More or less, it takes larger structures to get something that has a chance of 'not too bad' match over broader frequency range with a finite number of real components.


Glenn Elmore
 

Not claiming to be an expert but I would say that pattern is due to response in the far field.  As long as the 'matching' or other radiating mechanisms don't  have significant radiation compared to the 'antenna' then that response won't change.

Having said that, the *apparent* pattern can change. If mismatch gets high enough that other mechanisms, not intended to be included in what one calls the 'antenna', become significant compared to the desired/intended one then the pattern and performance can indeed change be different. 

This second situation is essentially what happens with common mode ingress. That which one *thinks* is the antenna is not really the total of what is providing response at the receiver.  Once intended antenna (mismatched) signals get down to the level of other signals, such as CM then the effective pattern is no longer entirely due to the intended antenna.

This problem can be aggravated by electrically small antennas which though they intercept the same aperture and, were they matched, would deliver the same signal power, deliver very small voltage or current levels due to their preamplifiers because of the low radiation resistance and high reactance and imperfect matching.

Perhaps surprisingly, were we able to perfectly match a [1 cm] dipole at 160m it would have just about the same performance as a full half wave. Of course we can't do that with materials we have, probably not even with super conductors so in practice this doesn't occur. 

But the general idea of using "something" as an antenna, no matter whether the feedpoint impedance is near something easy to access with reasonable matching materials, is perfectly fine as long as one compares the resulting mismatch to the target at a particular frequency of interest.  That target starts to get difficult at the high end of HF where we want to get increasingly close (or below) KTB. It's for this reason that either a loop, such as described by LZ1AQ, or my dipoles both fail to achieve the ITU 'quiet rural' noise floor at 10m.  More or less, it takes larger structures to get something that has a chance of 'not too bad' match over broader frequency range with a finite number of real components.


ON5KQ
 

I have a fundamental question - may be Glenn or other experts can answer:

I have a nice antenna in mind to build up for reception only (Wsprdaemon)
However this design is only well matched at a single band. Neglecting any radiating feedline or common-mode effects, the NEC2 model tells me however, that even with high mismatch  on other bands the radiation pattern would be different but also very beneficial.

Now for the practical construction, I wonder:
- if I would put a buffer pre-amplifier directly at the feedpoint (where the source in the NEC2 model is) and therefore do not introduce a feedline at all between antenna and pre-amp, would the radiation pattern will be any different than what the NEC2 models tells me, because of the heavy mismatch between the antenna impedance and the input impedance of the amplifier ? Would it require any special pre-amp to keep the radiation pattern unchanged to the NEC2 model ?

From theory I would say that there is no change in radiation pattern, rather than most likely a heavy drop in antenna efficiency (losses)... however from practical builds, my experience is completely different! Heavily mismatched antennas never really worked, even with a pre-amp...
(The design is for the high bands 10Mhz and higher)

As it would be some significant work to build it up - what do you think..?
Just go for it, even though it is a single band design and even though Wsprdaemon requires broadband performance ?
(but for reception with active feed)

What do I need to pay attention for, if I want to feed it with a pre-amp for wideband use, although the wire construction shows a narrowband characteristic at 75Ohm feedline (RG6)

Ulli, ON5KQ