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Old Sep 21, 2008, 11:31 PM
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Northern VA
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Modding junk 900 Mhz antenna

I took one of my many RV 900 mhz antenna and ripped it apart. I removed the plastic coating from the aerial to expose the spring like metal material. I took a picture of it, but it was pretty miserable so it isn't worth much. I plan on replacing the aerial part with a better length. The result should be very light and very easy to make. Also, everyone has these anteanns, so it is mostly free.

I used a good bit of heat to remove the spring, but thinking about it now, I may not have needed the heat at all. Anyway, I am left with a hole that I can place any length wire in and solder it up to create the proper length antenna. I will probably go with copper wire similar to Brian's dipole, except not a dipole.

Using antenna websites to find the proper length for 910 Mhz, it looks like a half wave is 6.1714", and a quarter wave 3.0857". The original antenna was about 3.5". Maybe we could just snip off a little to make a quarter wave antenna.

Does the 6.1714" sound like a good length? It appears that the real 1/2 wavelength at the speed of light is 6.485". The website length appears to be about 95% of the true speed of light.

Nathan
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Old Sep 21, 2008, 11:34 PM
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Northern VA
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I should have said that I took the screw out to work on the antenna. There are 2 tiny washers that go on either side of the aerial, so be careful not to loose those.
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Old Sep 21, 2008, 11:54 PM
my karma ranover my dogma
galaxiex's Avatar
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I made a 1/2 wave exactly the same way.
I just yanked the stock wire out and soldered in some heavy gauge solid copper wire that just fit the hole.

I used this calculator here...

http://www.crompton.com/wa3dsp/hamradio/antcalc.html

...and trimmed the wire as close as I could to the length from the above calculator.

It seems to work well, and the Tx doesn't get as hot as it used to.
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Old Sep 22, 2008, 12:04 AM
my karma ranover my dogma
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This calculator seems to give a result closer to the real 1/2 wavelength at the speed of light that you calculated as 6.485"

http://www.csgnetwork.com/freqwavelengthcalc.html
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Old Sep 22, 2008, 12:45 AM
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Quote:
It appears that the real 1/2 wavelength at the speed of light is 6.485". The website length appears to be about 95% of the true speed of light.
FWIW, using the speed of light for calculating antenna wavelength is inaccurate; The 95% correction factor accounts for the RF's velocity difference in a typical metallic medium (as opposed to free space propagation). So, the 6.485" should be something closer to 6.17".
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Old Sep 22, 2008, 06:03 AM
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Lewis Center, OH
Joined Jun 2006
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Where are you starting your measurement from? I can't tell how far the ground goes on the base, and depending on the orientation of the antenna I think it could be measured differently. Does the outer ground go beyond the SMA connector on these?

If you measure from the screw, I think the antenna could be too long. Especially if you have the antenna straight out. maybe since I've got a few of these I'll try to rip apart the base on one of them to see how far the ground goes out.

This is a great idea, though! I'm getting tired of dealing with that HUGE Hyperlinktech 3dBi antenna.
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Old Sep 22, 2008, 06:09 AM
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Hey guys,

Just a quick note here. If that transmitter was designed for a quarter wave antenna, then a half wave will be a very bad match. You might be able to match a 5/8 wavelength with a loading coil but you need a good SWR meter to get things right.

I have lots of those RV quarterwave antennas and found them to be only a fair to poor match. I may try triming one down to a better SWR if I get a chance.

OMM
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Old Sep 22, 2008, 06:42 AM
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Lewis Center, OH
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Quote:
Originally Posted by Old Man Mike
Hey guys,

Just a quick note here. If that transmitter was designed for a quarter wave antenna, then a half wave will be a very bad match. You might be able to match a 5/8 wavelength with a loading coil but you need a good SWR meter to get things right.

I have lots of those RV quarterwave antennas and found them to be only a fair to poor match. I may try triming one down to a better SWR if I get a chance.

OMM
Can you check to see if the the performance changes based on the orientation of the antenna?
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Old Sep 22, 2008, 07:06 AM
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oh no !
try to open the black plastic holder, you find what ?!?!
a coil ! and that is why your length measurements and calucations dont work,
but you are right the original antennas are known to have BAD swr,
you need access to RF measurement equipment to fix this most optimal
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Old Sep 22, 2008, 08:09 AM
my karma ranover my dogma
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It would be great if someone with the proper equipment would test these antenna and discover what length works best.
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Old Sep 22, 2008, 11:46 AM
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Amen.
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Old Sep 22, 2008, 04:34 PM
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Charles Town, WV
Joined Jan 2008
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I bet that coil is a inconsistant length explaining why some whips are better than others.
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Old Sep 22, 2008, 07:25 PM
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as a side note, another generic 900 mhz duck antenna i've worked with (not an RV one) had a bad connection at the swivel joint. it was only found when a buddy and i were doing looong range reception tests - too hard to see the effect at short distances. worked fine when fully extended (straight). it's made me leery of swivel/pivot antennas...

if you wanted to find the optimal antenna length without using a spec analyzer, one way you could do it is do a loong range test with a longer than needed antenna and slowly snip off length until you find the length with the best reception. you'll have to resolder another antenna of course, but then you'll know what length works at least subjectively. my guess is it's going to be hard to really see a big difference. would be more concerned with matching for the tx's sake.
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Old Sep 25, 2008, 12:41 AM
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Quote:
Originally Posted by Mr.Pibb
Can you check to see if the the performance changes based on the orientation of the antenna?
No sure of your question but if you asking about moving the antenna within the hinge, you get a little improvement at 90 degrees vs straight vertical. But the SWR is so bad in either case that I'm not surprised that people are having problems.

OMM
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Old Sep 25, 2008, 12:44 AM
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Quote:
Originally Posted by thomasscherrer
oh no !
try to open the black plastic holder, you find what ?!?!
a coil ! and that is why your length measurements and calucations dont work,
but you are right the original antennas are known to have BAD swr,
you need access to RF measurement equipment to fix this most optimal
Yep. Actually it forms an LC network because of the tight cavity around the coil. It is an interesting design and quite difficult to fully model.

OMM
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