On Jan 25, 11:25 pm, Bob Bob wrote:
Hi Ian
Sounds like you are having fun!
- I would suggest that using a GDO for meaningful antenna work at UHF
would incur quite a few inaccuracies. The big problem is how to couple
it to the element itself without detuning it by its presence. You will
hear some howls of protest but I personally wouldn't bother to try
making it a resonant length. It's really not that important for
radiation efficiency. The gamma match introduces a huge reactance that
also has to be allowed for.
- Using Ocarc's loss calculator 300r ribbon runs about 2.5dB for
100ft/30m. 450r open window ribbon runs about 0.7dB for the same
distance! (http://www.ocarc.ca/coax.htm) RG213 (approx 1/2") coax is
around 5dB. Keep in mind that ribbon bends and proximity to objects will
cause problems that coax wont.
- Converting coax to a balanced feed at the antenna can be very easily
done with a 4:1 coax balun. This is basically a extra loop of 1/2 wave
length coax built likehttp://www.n-lemma.com/calcs/dipole/balun.htmYou
will still need to match at the antenna in some way. Maybe replace the
gamma with a hairpin, use a delta match or maybe increase the number of
driven elements folded dipole style to get the Z up to 200 ohms. A yagi
feedpoint Z is going to be maybe 10-20r. One extra driven element
(simple folded dipole) will multiply that by 4 and a third element
another 4 times. That will be close enough to 200 ohms to work fine. You
can also play with the extra element diameters to vary the final feed Z.
- I personally wouldnt use a UHF TV balun without testing first. The
problem is quality of manufacture!
Hope you find this useful. Only some of my ideas!
Cheers Bob W5/VK2YQA
wrote:
To optimise the matching to the driven element I used a gamma
match, the reality is I don't know if the driven element is at resonant
frequency let alone correctly adjusting the gamma match.
I was just reading another posting on rec.radio.amateur.antenna,
where the author suggested using 300 ohm TV ribbon as it is less lossy
than a lot of coax at UHF (476 MHz). Is this true? 300 ohm ribbon will
remove the need for a gamma match at the antenna end, but at the
transmitter end an unbalanced to balanced ( 50 to 300 ohm) balun will
be required. Would a normal TV set balun suffice for 0.5 watt of
transmit power?
To resonate the driven element I have contemplated using a UHF
Gate Dip Oscillator (GDO), has anyone succeed in doing this with
minimal test equipment such as a GDO? If the folded driven element
resonates at 476 MHz I will accept that it's impedence is 300 ohms.- Hide quoted text -
- Show quoted text -
Bob,
Thank you for esponding to my questions, and apologis for my
delay in responding.
GDOs have their limitation as the coupling between the
oscillator and resonant circuit
causes the frequency to be pulled - a GDO with a frequency counter
might be a very useful instrument.
My main question about the use of a GDO is how high in
frequency can a practical GDO get
to in frequency and still oscillate, couple to a similar frequency
resonant system and be able to observe a
"dip".
Your advice on coupling and matching to the antenna are
excellent help, what I didn't understand
was the impedence value at the folded driven element. Probably
something from long ago, but I believed the
impedence of a folded element was 300 ohms???
Could you please explain what you meant by :-
"or maybe increase the number of
driven elements folded dipole style to get the Z up to 200 ohms. A yagi
feedpoint Z is going to be maybe 10-20r. One extra driven element
(simple folded dipole) will multiply that by 4 and a third element
another 4 times."
In physical term I don't know what this means.
An article that caught my attention recently that I will share
with you is a cheap method
for measuring complex impedance, it is at:-
http://web.ukonline.co.uk/g3ldo/aegextra.htm
Once more thanks for your help.
Regards,
Ian