On 21 mar, 16:38, John - KD5YI wrote:
On 3/21/2011 7:06 AM, Wimpie wrote:



On 21 mar, 00:42, John - *wrote:
On 3/20/2011 5:11 PM, Wimpie wrote:

On 20 mar, 21:09, * *wrote:
On Mar 19, 5:59 pm, * *wrote:

On 19 mar, 22:25, * *wrote:

I was palnning on building a 2M Yagi this weekend and was wondering if
my available bill of materials would have a significant effect on
normal design parameters. The plan is to use 3/4 inch EMT conduit for
the boom and 3/8 inch all-thread for the elements. My concern was that
the threads could change the design frequency.

JImmie

Hello Jimmie,

I think it will be OK, but if you want to be 100% sure, make a dipole
of same thickness tubing and one from the threaded material of choice.
If results are same, you can use it without dimension scaling. *Are
you using galvanized or alu material?

If required, apply some corrosion protection.

With kind regards,

Wim
PA3DJSwww.tetech.nl
.

Galvanized, The pupose was to demostrate *that materials can often be
adlib and still have a useable antenna. Maybe this one will not last
30 years before it corrodes away but probably 5 or 6. Also I want to
demonstrate the value of using a jig when building many antennas all
made the same. I have tried the test you mention on 10 meters before
replacing a vertical mounted on my truck. Changing materials didn't
seem to make much difference. WIll *try the same for 2M but I dont
have an SWR meter for 2M at this time.. Was hopng someone in the group
might think this interesing enough to try.

Jimmie

Hello Jimmie

Zinc has about 60% conductivity of alu alloys that are used for
antennas, so the surface resistance will be almost 80% of alu tubing
(same diameter). *Of course you have thread that increases the path
length. Given the thread, your net conductivity will be about 40%
(w.r.t. alu).

This will be OK for doing your experiments and to demonstrate that you
can use many materials for antennas. I would recommend you to find
some means to determine VSWR (or refl. coeff.) at 2m (home made
slotted coaxial line, or home brew coupler?).

With kind regards,

Wim
PA3DJS
www.tetech.nl
Without abc, PM will reach me

Here is one the OP can build:

http://www.qsl.net/xq2fod/Electron/swr/swr.html

Hello John,

I made (and still use) same type of wheatstone bridge refl. coeff.
meter with BAT15 diodes (so only low power) and 0805 SMD. Instead of a
moving coil indicator I used a 10 MOhm DVM (to get better linearity at
low power input).

That's a great idea. I wish I had thought of that.

John

Hello John, Jimmie and others,

If the DC input resistance of the DVM is very large compared to the
zero bias differential resistance of the diode, there is a tight
relation between DC output voltage and the RF input voltage with
reasonable temperature behavior.

If you measure the RF to DC curve at about 5 RF input levels (at say
10 MHz, just with an oscilloscope and simple RF signal source), you
can make a curve fitted table/graph to handle the linear to quadratic
transition at around 150mV RF peak voltage. This enables you to
convert DC output to RF input voltage. You can expect about 1mV DC
from a real schottky diode for 10mVp RF input voltage (across the
diode junction).



Wim
PA3DJS
www.tetech.nl
without abc, PM will reach me