On Sat, 09 Jun 2012 19:47:03 +0100, Jeff wrote:
On 09/06/2012 17:53, Jeff Liebermann wrote:
On Sat, 09 Jun 2012 09:45:30 +0100, wrote:
Put a sample in the microwave and see if it get hot, it may be an
insulator but it also may be lossy at rf.
Jeff
That's a good way to determine if it will work at 2.4GHz. However,
this is for HF, which is about 1/500th of 2.4GHz. Some insulators
that fail at 2.4Ghz (PVC, nylon, phenolic, wood, bakelite), work just
fine at HF frequencies. In general, if it passes at 2.4GHz, it will
probably work at HF. However, if it fails, it may still work at HF.
Loss tangent does not generally vary much with frequency unless there is
a lot of water content or similar, that is to say that it is linear
against frequency, so it is not a case of pass or fail, it is just how
hot does it get when scaled to HF.
Loss tangent (dissipation factor) can increase or decrease with
frequency depending on material. It's measured at 1KHz, but can be
almost anything by the time it gets to microwave frequencies. For
example, glass:
http://psec.uchicago.edu/glass/tanDelta_vs_freq_edited.bmp
There's also the potential problem of volatiles and solvents. Some of
the components in the body putty are hygroscopic (absorbs moisture).
Well if they do they will not be good even at HF.
Time to define "good". The OP proposes to use Bondo at the low
impedance end of a gamma match. That's 50 ohms. It would
considerable conductivity to put anything in parallel with 50 ohms to
have an effect.
Even placing an obvious conductor between the 50 terminals isn't going
to do much. For example, I could place a water balloon full of salt
water between the 50 ohm terminals. As long as rubber provides
sufficient insulation, there will be no dielectric loss. It might arc
over or add some capacitance depending on size, but it won't conduct
RF.
However, were he to propose using Bondo at the junction of the
matching tap section and the tuning capacitor where it hits the driven
element, there will be problems. The gamma match is a series resonant
tuned circuit at this point with high voltage across the capacitor. In
effect, the Bondo would act as part of the capacitor dielectric at
high voltages. Bondo is not suitable for building capacitors.
When baked in a microwave oven, the water and solvents may boil,
resulting in cracking or crumbling.
Not a problem if it is just a test sample, but if they absorb water they
will be a poor insulator.
Not exactly. A water soaked insulator is certain a potential problem.
Yet, there are perfectly usable insulators that contain water. For
example, concrete, bricks, and some pottery are contain considerable
water, but are tolerable insulators, especially if glazed. HF has no
effect on the water because there is insufficient energy at 2-30MHz to
convince the water molecule to vibrate. The higher the frequency, the
higher the energy:
E = Plank's-constant * speed-o-light / wavelength
While building an HF antenna structure out of bricks is feasible, I
would not suggest you place a brick in a microwave oven. It will get
hot and eventually crumble as the water evaporates. For a higher
speed example, just hit a brick or concrete block with a oxy-acetylene
torch and be prepared to duck as pieces fly out as the water explodes.
Residual ethanol solvent might
make an interesting bang.
I think we must be thinking about a different type of filler, all of the
glass fibre fillers I have come across do not contain ethanol, they are
2 part; styrene monomer and organic peroxide hardener
Please re-read my summor of the contents from the MSDS sheets. The
stuff is 30-60% ethanol solvent by weight. That's needed to keep the
mix flexible and compliant. As it evaporates, the Bondo hardens.
However, if some residue remains, such as the result of applying the
Bondo in too thick a layer, there is a real possibility of a gas
explosion.
Jeff
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558