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Old Nov 01, 2010, 09:57 PM
Destructive testing expert!
Bill Sanford's Avatar
Bemidji, MN
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Anyone tried building a helical antenna for FPV?

Hi Guys,

Has anyone tried to build a helical antenna for their FPV base station?

Helicals are circular polarized due to their design and this has many advantages for our application. Receive signal strengths are much more consistent when the plane banks.

Here's a starting point:
http://www.dxzone.com/cgi-bin/dir/jump2.cgi?ID=23560

I need to buy an ARRL Antenna Handbook. I've been a ham for many years and borrowed a friend's book for a few projects many years back. It may be time to invest in one for this aspect of the RC hobby!

Who will be the first to try this design? I'm guessing that someone already has...

Bill
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Old Nov 01, 2010, 10:15 PM
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This looks very interesting from the web site I mentioned.

Good gain, wide pattern and CP!

Try plugging in numbers on the web site. It's easy to get quite a bit of gain from this design to go really far out... The best part is no polarization issues.
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Old Nov 01, 2010, 11:02 PM
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Helical Antenna

Circular polarization is the only way to go and Helical antennas are one good way to get it.
Not using CP is one of the reasons guys are losing video on long range flights. They fly out to the limits until the picture gets noisy which on an FM system is just a couple of db from lights out. Bank the plane and the picture is gone.

Here is a picture of one a friend of mine is building for his setup. It has a few bugs which he has to work out. but it will help him break the long distance record (which he set) for round trip fpv flight.

73
Cliff K.

BTW If you are interested in 23 cm video, check out my review of Sunsky system on the 1.2GHZ TX/RX All Infos on One Page site(or something like that), posts #411,412, and 414.
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Old Nov 01, 2010, 11:53 PM
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Is that antenna as big as it looks? ;P
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Old Nov 02, 2010, 03:14 AM
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I have a few built and tuned (spec an with VSWR brudge), they work well, not as convenient and portable as a flat patch though.

Cheers,

Sander.
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Old Nov 02, 2010, 03:16 AM
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Quote:
Originally Posted by cliffkot View Post
Circular polarization is the only way to go and Helical antennas are one good way to get it.
Not using CP is one of the reasons guys are losing video on long range flights. They fly out to the limits until the picture gets noisy which on an FM system is just a couple of db from lights out. Bank the plane and the picture is gone.

Here is a picture of one a friend of mine is building for his setup. It has a few bugs which he has to work out. but it will help him break the long distance record (which he set) for round trip fpv flight.

73
Cliff K.

BTW If you are interested in 23 cm video, check out my review of Sunsky system on the 1.2GHZ TX/RX All Infos on One Page site(or something like that), posts #411,412, and 414.
Ps. take notice that the former you use to wind the helical on *will* shift your frequency down, as its dielectric properties become part of the antenna design. If you need me to get into further detail, just say so and I'll outline exactly what happens.

Cheers,

Sander.
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Old Nov 02, 2010, 06:12 AM
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I have been considering helical antennas for some time now, but have decided to go with a circular patch simply because it is compact.

Helicals are fairly easy to build, but it's finding a good form to wind around that's the problem. Of course, as Sander points out, if you know the dielectric constant of the material, you can calculate the frequency shift and compensate for it.

-Alex
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Old Nov 02, 2010, 08:12 AM
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Reading the post title I got drawn as I thought someone was going to describe a CP antenna for the Tx. Better still, an omni directional CP solution.

Still interested to see what is being done on the Rx side of things.

Nigel.
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Old Nov 02, 2010, 06:24 PM
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Quote:
Originally Posted by ssassen View Post
Ps. take notice that the former you use to wind the helical on *will* shift your frequency down, as its dielectric properties become part of the antenna design. If you need me to get into further detail, just say so and I'll outline exactly what happens.

Cheers,

Sander.
Yes, it will, but PVC pipe is not really necessary. I think Crist got a little carried away. He was building a bazooka! I think one about 24 inches long would be a good place to start.

Cliff K.
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Old Nov 02, 2010, 06:29 PM
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Quote:
Originally Posted by Devonboy View Post
Reading the post title I got drawn as I thought someone was going to describe a CP antenna for the Tx. Better still, an omni directional CP solution.

Still interested to see what is being done on the Rx side of things.

Nigel.
CP helical at current frequencies would be a bit unwieldy at the VTX end of things and is not necessary. As long as you use it at the receive end, you can maneuver the airplane any which way and receive the same signal as long as the planes antenna is reasonably omni. There are blind spots of course caused by the VTX antenna pattern, but it is then not a polarization issue. You can't fix everything, only the obvious flaws.

Cliff K.
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Old Nov 03, 2010, 03:39 AM
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Cliff, agreed.

My interest is primarily at 5.8GHz and omni directional CP at the VTx to provide minimal multipath issues at the VRx. Also, 2 fliers could be at the same field, one with RHCP and the other with LHCP and potentially not interfere with each other.

Nonetheless, this is still an interesting thread to further explore the VRx end of things.

Nigel.
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Old Nov 03, 2010, 10:14 AM
FPV is my cup of tea
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"Left/Right circular polarized antenna ?" might be an interesting read.
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Old Nov 03, 2010, 10:20 AM
FPV is my cup of tea
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Originally Posted by Devonboy View Post
Also, 2 fliers could be at the same field, one with RHCP and the other with LHCP and potentially not interfere with each other.
True, but their TX antennas must be fully CP in the direction of the ground station.
If one banks more than 90 degrees, probably much less, it will interfere.

This is due to the fact that an omni directional CP is RHCP in one direction and LHCP in the other.
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Old Nov 03, 2010, 10:22 AM
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In (just for the education).
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Old Nov 03, 2010, 10:23 AM
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Bemidji, MN
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Quote:
Originally Posted by cliffkot View Post
Yes, it will, but PVC pipe is not really necessary. I think Crist got a little carried away. He was building a bazooka! I think one about 24 inches long would be a good place to start.

Cliff K.
Yea, I chuckled when I saw Crist's antenna. He must be serious about going for the distance record. A helical with that many turns has over 20db of gain!
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