Capacitors for HF Antenna
 

Hi all,
I am trying to do a loop antenna (HF - 13.56 Mhz - RFID Application),
I found people using air variable capacitors and mica variable
capacitors.
can i use ceramic capacitors ? will that affect the performance a lot ?
will that spoil the Q of antenna a lot ?

regards,
palani

Capacitors for HF Antenna
 

palaniappan chellappan wrote:

Hi all,
I am trying to do a loop antenna (HF - 13.56 Mhz - RFID Application),
I found people using air variable capacitors and mica variable
capacitors.
can i use ceramic capacitors ? will that affect the performance a lot ?
will that spoil the Q of antenna a lot ?

regards,
palani


Are ceramic variables available??

The reason variable capacitors are used is that variables can tune the loop to
resonance.

Capacitors for HF Antenna
 

ORIGINAL MESSAGE:

On 24 Jul 2006 00:45:39 -0700, "palaniappan chellappan"
wrote:

Hi all,
I am trying to do a loop antenna (HF - 13.56 Mhz - RFID Application),
I found people using air variable capacitors and mica variable
capacitors.
can i use ceramic capacitors ? will that affect the performance a lot ?
will that spoil the Q of antenna a lot ?

regards,
palani

------------ REPLY SEPARATOR ------------

You didn't give many details such as the bandwidth of the antenna, but
if you use the NP0 type ceramic capacitors you should be ok. Loops,
especially small ones, are notorious for narrow bandwidth and stable
caps are a must.

Bill, W6WRT

Capacitors for HF Antenna
 

Bill Turner wrote:
You didn't give many details such as the bandwidth of the antenna, but
if you use the NP0 type ceramic capacitors you should be ok. Loops,
especially small ones, are notorious for narrow bandwidth and stable
caps are a must.

I once used disc ceramic bypass caps to try to tune an
antenna. They went up in flames. For a small loop, I
would at least use doorknob quality caps.
--
73, Cecil http://www.qsl.net/w5dxp

Capacitors for HF Antenna
 

Cecil Moore wrote:
Bill Turner wrote:
You didn't give many details such as the bandwidth of the antenna, but
if you use the NP0 type ceramic capacitors you should be ok. Loops,
especially small ones, are notorious for narrow bandwidth and stable
caps are a must.

I once used disc ceramic bypass caps to try to tune an
antenna. They went up in flames. For a small loop, I
would at least use doorknob quality caps.
I would also look for RF caps with a KVAR ratings.

Most caps even doorknobs are just simple HV power supply doorknobs with
no RF ratings. Ebay is full of this stuff, most of them are useless at HF.

Many of these caps have a resonance dip in them. I have found many that
dip at 7,8 and 13mhz. When i applied power you can figure out what
happened! So check them with a dipper. Good caps like HT57s will have
no dips. Some of the Russian pancake types are also good, but most are
GP high voltage caps for laser power supplies.

The ultimate caps are Tubular Types made by people like Morgan, however
they would make wire antenna with caps as expensive as a Steppir!
I was lucky enough to find a few of these for cheap and used them on a
N6LF type improved zepp antenna. These caps were rated at 50 KVar.
Someone told me 1kvar is about a kilowatt.


Pat

Capacitors for HF Antenna
 

Cecil Moore wrote:
Bill Turner wrote:
You didn't give many details such as the bandwidth of the antenna, but
if you use the NP0 type ceramic capacitors you should be ok. Loops,
especially small ones, are notorious for narrow bandwidth and stable
caps are a must.

I once used disc ceramic bypass caps to try to tune an
antenna. They went up in flames. For a small loop, I
would at least use doorknob quality caps.

Thanks to all for replies,
Actually my requirement is to pump 4W into 1meter diameter loop
antenna.
I need to tune the antenna to resonance 13.56Mhz and then match it.
I using some fixed capacitors and variable capacitors of small range
for fine tuning.
Now i am using ceramic fixed and ceramic variable capacitors and found
that its performance is good (I am not sure, the problem may in my
transceiver also).

I thinking of trying other capacitors, how about following combinations
mica fixed + mica variable
mica fixed + air variable

Is it worth to buy mica capacitors ?
can i simply buy mica capacitors ? , i found some vendors selling rf
mica capacitors ,
is there any big difference between them for HF frequency ?

regards,
palani

Capacitors for HF Antenna
 

I thinking of trying other capacitors, how about following combinations
mica fixed + mica variable
mica fixed + air variable

Another option is to homebrew your own capacitors. I've seen
descriptions of magloop-antenna tuning caps of a number of interesting
sorts which are fairly easy to homebrew:

- A sheet of glass as the dielectric, with metal plates (or the
surface copper of sheets of PC-board material) as the electrodes.
These can be made variable by arranging to slide one
plate-electrode over, or away from, the other.

- Fixed caps made of PC-board material (etch away a strip around the
edge of each side to prevent arcing)

- "trombone" caps made of concentric copper or brass pipe or tubing,
with Teflon or polyethylene film rolled onto the inner tubing as a
dielectric.

If you only require single-frequency operation you might be able to
build a metal-and-glass or PC-board fixed cap which has more capacity
than you need, and them trim it (literally) by removing a bit of the
metal at a time until you reduce the capacity to the value which
establishes resonance at your operating frequency.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

Capacitors for HF Antenna
 

ORIGINAL MESSAGE:

On Mon, 24 Jul 2006 18:50:41 GMT, Cecil Moore
wrote:

I once used disc ceramic bypass caps to try to tune an
antenna. They went up in flames. For a small loop, I
would at least use doorknob quality caps.
--

------------ REPLY SEPARATOR ------------

He was asking about an RFID application, presumably a few milliwatts.

Bill, W6WRT

Capacitors for HF Antenna
 

Maxwell Smart #99 wrote:
Someone told me 1kvar is about a kilowatt.

There's no real power in a Var since Vars = V*I*sin(A).
--
73, Cecil http://www.qsl.net/w5dxp

Capacitors for HF Antenna
 

Bill Turner wrote:
He was asking about an RFID application, presumably a few milliwatts.

I think he said 4 watts to a one meter loop. Even in that
application, I suspect the physical flexing of ceramic caps
would cause losses. After mine caught on fire, I found out
that at least some ceramic caps are designed to be biased by DC.
--
73, Cecil http://www.qsl.net/w5dxp