Modifying a discone antenna

[tony]

Hi,

I need a wideband antenna for EMC compliance testing. A discone is
wideband 25-1300 MHz, lousy gain - about 0 dB I am told, so should be
fine as a general purpose antenna BUT it is omnidirectonal.

Can I remove some of the radials to make it more directional?

And, if so, can anyone tell me what sort of improvement in rejection of
unwanted (ie side and back) signals I will get?

I would guess that the major issue is change of impedance, but I do not
know enough (and cannot find enough) about discones to verify this.


Many thanks,

Tony

 

[Rod Speed]

I need a wideband antenna for EMC compliance testing. A discone
is wideband 25-1300 MHz, lousy gain - about 0 dB I am told, so should
be fine as a general purpose antenna BUT it is omnidirectonal.

Can I remove some of the radials to make it more directional?

Wont help on that.

And, if so, can anyone tell me what sort of improvement in
rejection of unwanted (ie side and back) signals I will get?

Sweet **** all.

I would guess that the major issue is change of impedance,

Nope, that wont change significantly.

but I do not know enough (and cannot
find enough) about discones to verify this.

They're so simple that you dont see much discussion of them.

 

[Malcolm Faed]

Hi Tony,

Perhaps you could try a log periodic like this
http://www.grove-ent.com/ANT16.html

Regards,
Mal.

 

[tony]

Hi Mal,

Thanks for the reply.

I tried to find a log periodic - I left it a bit late and can only get
one ordered in to arrive in a week or so, but I am doing the EMC testing
this week ..

I have got a discone though, so am looking at what options I have with
just it and a long piece of string (for the lower frequencies).

One option with that is a shield (we have welding screens made of 1.2mm
sheet steel (a sheet about 1.5m x 2.4m in a metal frame). Things like
reflections and standing waves spring to mind, but I am assuming that if
I physically move the screen I can pretty quickly sort out what is a
reflection or standing wave. I am also grounding the screen at a number
of places to try and reduce reflections.


Thanks,

Tony

 

[smithxpj]

Hi,

I need a wideband antenna for EMC compliance testing. A discone is
wideband 25-1300 MHz, lousy gain - about 0 dB I am told, so should be
fine as a general purpose antenna BUT it is omnidirectonal.

Can I remove some of the radials to make it more directional?

And, if so, can anyone tell me what sort of improvement in rejection of
unwanted (ie side and back) signals I will get?

I would guess that the major issue is change of impedance, but I do not
know enough (and cannot find enough) about discones to verify this.


Many thanks,

Tony

If you're into *genuine* compliance testing then the moment you start
fiddling around with antenna modifications then any meaningful
calibration will have gone out the window. Compliance means that you
*have* to test to a known standard.

The reason that manufactured EMC antenna kits come at a price is
because of the known precision and the calibration figures provided
for each individual antenna.

You can certainly take a stock discone and chop and change the bits
but then is it worth getting it calibrated versus buying an antenna
specifically intended for RF testing in a realm that requires accurate
measurements against specified standards?

 

[tony]

Hi,

Thanks for the reply.

I have been out of it for the last week, but got better without drugs.

The initial tests are only for comparative results ... we want to do
pre-compliance testing and noise source isolation etc before spending
the big dollars on a lab. Also our product is built into a sea container
so shipping it to a compliance lab (from Perth) is going to be expensive
as well.

For this directivity and relative measurements are more important than
absolute measurements.


Thanks,

Tony