EMRFD Message Archive 9640

Message Date From Subject
9640 2014-01-06 19:43:17 Ashhar Farhan electrical steppir
this a little out of step from what we discuss at emrfd. but i guess antenna, tuners are relevant here too.

consider a rotatable dipole of 51 feet span with a balanced tuner mounted right at antenna. this should tune most of the HF bands.

now consider a second element mounted at let's say 12 feet distance from the 'driven element'. and we add a 'sort-of' antenna tuner to this element too. 

will it be possible to tune both the antenna tuners such that :
A. we can tune the entire system to cover wide bands (let's say  7 to 30 Mhz)
B. gain directionality.
C. Switch the elements such that the beam could flip back-to-front quickly.

while i understand antennas nominally, i am not sure of what the passive element's tuner should look like. will just variable reactance in the center of the element be enough?

sorry if this seems a little purile. as i said, i have only slung out dipoles all my life. it is only that last month my friend VU2PEP (Paddy) helped me put up a two element yagi for 20. I am now looking for a short antenna that is steerable over a wide-range. someone pointed me to steppir ...

- f
9641 2014-01-07 08:13:53 AD7ZU Re: electrical steppir
Farhan,
A good way to verify your ideas is to model the system.  There are several antenna/feedline modeling tools available but most are expensive if you get the versions with full capability, however 4NEC2 is freeware and uses the same NEC core as EZNEC and others.
you can download it here:  http://www.qsl.net/4nec2/
there are sweep and optimization functions which could be used to verify the range of LC components in both the driven and parasitic element(s).  The software is not as polished as the commercial packages but is very usable ..it just takes some time to learn to drive.
I have successfully used it for several applications.
Hope that helps,
 
Randy
AD7ZU


9642 2014-01-07 08:25:58 Dan Rae Re: electrical steppir
A project like this authored by g3xjp under the title "Pic-A-Yagi" or
something similar, was published in the RSGB magazine Radcom maybe a
year ago... I'm sure google will unearth it and there is probably a
Yahoo group as well.

73 - Dan
9643 2014-01-07 15:45:44 kb1gmx Re: electrical steppir

Yes, but the 51ft element would be problematic on bands above 30m.  Once the elemnts get longer than

1.25 lambda  the impedence  to match are complex and high. and the pattern is not dipole like.  The idea can work to a point. 


STEPIR is a variable length antenna, accomplished by moving bands of Phosphor-Bronze which is both a good conductor and also able to flex without failure.


You can build a yagi with loaded elements (electrically short) but the gain is a slight bit lower and there

is a practical limit to how much shortening a two element yagi without loosing gain and pattern.  Shorter 

than half size  really hurts gain.  Also shortening the boom length has limits and for useful gain .15 to .25

lambda is considered the sweet spot.


Interesting idea but without adjustable length elements the range of frequencies are limited though a 2:1 range of tuning is doable.  Then again a trapped beams of three elements for 20/25/10 exist.  Doing it 

with a flexible  wire, stepper motors and pulleys has been done and is known to work.  Wire beams 

work so one with adjustable wires would work as well.


The another approach is a wire log periodic.  



Allison

9644 2014-01-07 15:57:34 William Carver Re: electrical steppir
The PIC-a-yagi HAS variable element lengths. Unlike SteppIR, Peter has
tubing pulled in/out with a cable loop and screwdriver-type motors.
Controlled, as the name indicates, with a PIC micro.

W7AAZ


On Tue, 2014-01-07 at 15:45 -0800, kb1gmx@arrl.net wrote:
>
> Yes, but the 51ft element would be problematic on bands above 30m.
> Once the elemnts get longer than
>
> 1.25 lambda the impedence to match are complex and high. and the
> pattern is not dipole like. The idea can work to a point.
>
>
> STEPIR is a variable length antenna, accomplished by moving bands of
> Phosphor-Bronze which is both a good conductor and also able to flex
> without failure.
>
>
> You can build a yagi with loaded elements (electrically short) but the
> gain is a slight bit lower and there
>
> is a practical limit to how much shortening a two element yagi without
> loosing gain and pattern. Shorter
>
> than half size really hurts gain. Also shortening the boom length
> has limits and for useful gain .15 to .25
>
> lambda is considered the sweet spot.
>
>
> Interesting idea but without adjustable length elements the range of
> frequencies are limited though a 2:1 range of tuning is doable. Then
> again a trapped beams of three elements for 20/25/10 exist. Doing it
>
> with a flexible wire, stepper motors and pulleys has been done and is
> known to work. Wire beams
>
> work so one with adjustable wires would work as well.
>
>
> The another approach is a wire log periodic.
>
>
>
> Allison
>
>
>
9645 2014-01-07 17:38:12 Brooke Clarke Re: electrical steppir
Hi Ashhar:

You might consider a time delay beam steering antenna.  It's a more general version of a classic phased array antenna.
The problem with the phased array is that the beam azimuth changes with frequency but with time delay beam steering the azimuth is independent of frequency.
This system requires 2 or more wide band antennas.  Fenwick who patented the system also patented a multi-band antenna that has vertical polarization on the lower ham bands and horizontal polarization