I'm one of several adult mentors to a local high school ham club that's been designing and flying balloons. We are designing a high speed 70cm digital downlink for a future payload. One of my major concerns is frequency selective multipath fading in the 1 MHz bandwidth I plan to use as the balloon gets low on the horizon of the tracking station and the receiver begins to pick up a reflection off the ground.
I think one of the simpler ways to deal with multipath is to use circular polarization on both ends. This automatically rejects all odd-order reflections (including the all-important 1st order reflection off the ground) as they have the opposite circularity sense to the direct signal and automatically be cancelled at the receiver. (CP was tried experimentally with analog TV broadcasting back in the 1970s as a means of reducing ghosting.)
On the ground we'll just use a regular 70cm satellite antenna, but on the balloon I need a 70cm circularly polarized transmit antenna with a fairly wide (hemispherical) beamwidth that can maintain its circularity over as much of that as possible.
I'm not an antenna guy, but my understanding tells me that a quadrifilar might be a good choice here. Any comments on this? Can anybody point me to some typical performance figures for these antennas? Again, I'm more interested in cross-polarization rejection than in absolute gain. I have plenty of link margin and I'm willing to sacrifice a few dB if necessary to get rid of those deep multipath fades.
Thanks,
Phil
Hi Phil,
QFH is effectively circularly polarized, it is often used for GPS receivers, since when you point it skywards the main lobe points trough the zenith, while nulls are pointing to the horizon (where you do not have satellites normally).
In my opinion QFH risks to be quite bulky for a balloon, since it will occupy a cylinder 1 wavelength high (70cm in your case).
To keep it simple, I would rather go for a pair of crossed dipoles 90° phased over a ground plane: the crossed dipoles will be fairly larges (about half wavelength, 35cm) but light since it is made of some sort of rigid wire. The reflector can be the bottom side of the payload, if the latter is cubic-shaped and it's at least 35x35cm (otherwise, you can use a metal net and fix it to the bottom of the payload.
The crossed dipoles will be 1/4 of wavelength spaced from the reflector, that is 17.5cm in free space, but can be reduced if you insert some sort of dielectric (like Styrofoam).
The gain will be essentially an hemisphere, if you need to reduce it on the horizontal direction, just enlarge a bit the reflector, and bend it slightly trough the dipoles.
Unfortunately I do not have any figures on hand for the cross-polarization performances, but IMHO, this is the best complexity-performance compromise.
73s
Simone - IU1AFY (ex. IW1FYV)
2013/7/17 Phil Karn [email protected]
I'm one of several adult mentors to a local high school ham club that's been designing and flying balloons. We are designing a high speed 70cm digital downlink for a future payload. One of my major concerns is frequency selective multipath fading in the 1 MHz bandwidth I plan to use as the balloon gets low on the horizon of the tracking station and the receiver begins to pick up a reflection off the ground.
I think one of the simpler ways to deal with multipath is to use circular polarization on both ends. This automatically rejects all odd-order reflections (including the all-important 1st order reflection off the ground) as they have the opposite circularity sense to the direct signal and automatically be cancelled at the receiver. (CP was tried experimentally with analog TV broadcasting back in the 1970s as a means of reducing ghosting.)
On the ground we'll just use a regular 70cm satellite antenna, but on the balloon I need a 70cm circularly polarized transmit antenna with a fairly wide (hemispherical) beamwidth that can maintain its circularity over as much of that as possible.
I'm not an antenna guy, but my understanding tells me that a quadrifilar might be a good choice here. Any comments on this? Can anybody point me to some typical performance figures for these antennas? Again, I'm more interested in cross-polarization rejection than in absolute gain. I have plenty of link margin and I'm willing to sacrifice a few dB if necessary to get rid of those deep multipath fades.
Thanks,
Phil
Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
On 2013.07.17 10:11, Simone wrote:
Hi Phil,
QFH is effectively circularly polarized, it is often used for GPS receivers, since when you point it skywards the main lobe points trough the zenith, while nulls are pointing to the horizon (where you do not have satellites normally).
In my opinion QFH risks to be quite bulky for a balloon, since it will occupy a cylinder 1 wavelength high (70cm in your case).
QFH for 70cm band is 264 mm height only (and 84 mm in diameter).
73!
I must have screwed up with units somewhere...
Simone - IU1AFY/F4VPY
2013/7/17 Zilvinas, LY2SS [email protected]
On 2013.07.17 10:11, Simone wrote:
Hi Phil,
QFH is effectively circularly polarized, it is often used for GPS receivers, since when you point it skywards the main lobe points trough the zenith, while nulls are pointing to the horizon (where you do not have satellites normally).
In my opinion QFH risks to be quite bulky for a balloon, since it will occupy a cylinder 1 wavelength high (70cm in your case).
QFH for 70cm band is 264 mm height only (and 84 mm in diameter).
73!
______________________________**_________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/**listinfo/amsat-bbhttp://amsat.org/mailman/listinfo/amsat-bb
Hi Phil, I've flown a couple of balloons as an advisor to an undergraduate lab here at Virginia Tech. We use CP on all of our flights, though generally we use the standard 2m APRS freq and not 70cm. Our first two flights we flew a crossed dipole suspended above the payload (to get it out of the view of the down facing camera) and had excellent results. Our second flight flew the Byonics MicroTrak 300, with only 300 mW of power and had no problems hitting digipeaters and iGates from 100,000+ feet. The crossed dipole was constructed out of aluminum arrow shafts (rigid, but lightweight). In an effort to drop even more weight, we changed to a design that uses fairly rigid wire in a "turnstiled loop" configuration, more commonly known as an eggbeater, again with excellent results from flight. In both cases we decided to forego installing the ground plane mentioned in previous posts. This was primarily a design decision made to save weight at the loss of a few dB (energy radiated up does us no good with receivers on the ground). One design that I have wanted to try but haven't yet had the opportunity is the crossed moxon. Essentially, it is like taking the reflector and driven elements of a pair of yagis and crossing them at 90 degrees. It is slightly different from the Yagis as the end of the reflector is bent 90 degrees toward the driven and the driven ends are bent 90 degrees towards the reflector (for impedance manipulation). L.B. Cebik, W4RNL, wrote a very useful article in the August 2001 edition of QST describing the construction of the crossed moxon, complete with design dimensions for a 435.6MHz crossed moxon (google can direct you to a copy of his article). I do not have information about the cross-pole rejection, but some time spent with 4NEC2 antenna modelling software may give you a bit of insight. With the bends in the elements of the moxon it focuses the pattern a bit more towards the horizon (over the crossed dipoles). I'm not sure about your location, but for us in SW Virginia our balloons get whipped right along in the jet stream and we have trouble keeping up with them during the chase (peak lateral speed in the Jetstream for our flights was 143 MPH). The result of this is that at no time (other than the initial release when the balloon is under about 10,000 ft) does the balloon get much higher than maybe 20 degrees elevation. Having energy focused more towards the horizon and less towards zenith (both in the balloon antenna and the ground antennas) is probably not a bad Idea in our case.
If you search callsigns KK4MOB and KK4PWM on aprs.fi you can see the path of our flights and the insanely high speeds in the Jetstream (around 30k-40k ft).
Hope the responses help inform your decision. GOOD LUCK!
-Zach, KJ4QLP
-----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Zilvinas, LY2SS Sent: Wednesday, July 17, 2013 6:33 AM Cc: AMSAT BB Subject: [amsat-bb] Re: Help wanted on CP antenna design
On 2013.07.17 10:11, Simone wrote:
Hi Phil,
QFH is effectively circularly polarized, it is often used for GPS receivers, since when you point it skywards the main lobe points trough the zenith, while nulls are pointing to the horizon (where you do not have satellites normally).
In my opinion QFH risks to be quite bulky for a balloon, since it will occupy a cylinder 1 wavelength high (70cm in your case).
QFH for 70cm band is 264 mm height only (and 84 mm in diameter).
73! _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Hi Zilvinas, LY2SS
I agree completely with you because the QFH was used for the beacon of OSCAR-7 at 2304 MHz and in addition to be endfire radiation or bakfire radiation it do not require a reflector.
The size of a QFH for 70 cm is very small as you quoted in 264 mm height only and 84 mm in diameter and so it is not very big and weight not to much for a balloon particularly because there is not a reflector in it.
I hope that you as well Phil KA9Q and Simone IU1AFY have received my pdf file concerning the QFH written by Walter Maxwell W2DU and another one for 435 MHz written by my self in italian for Radio Rivista 10/1998
I suggest you to read also my article:
"Experimental investigation of Quadrifilar Helix Antennas" published by the AMSAT Journal May/June 2004
Best 73" de
i8CVS Domenico
----- Original Message ----- From: "Zilvinas, LY2SS" [email protected] Cc: "AMSAT BB" [email protected] Sent: Wednesday, July 17, 2013 12:32 PM Subject: [amsat-bb] Re: Help wanted on CP antenna design
On 2013.07.17 10:11, Simone wrote:
Hi Phil,
QFH is effectively circularly polarized, it is often used for GPS receivers, since when you point it skywards the main lobe points trough the zenith, while nulls are pointing to the horizon (where you do not
have
satellites normally).
In my opinion QFH risks to be quite bulky for a balloon, since it will occupy a cylinder 1 wavelength high (70cm in your case).
QFH for 70cm band is 264 mm height only (and 84 mm in diameter).
73! _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Hi Domenico,
I received and briefly went trough both articles.
Seeing the front cover scan of Radio Rivista with the orange pipe brought back to my mind that I read that article from the magazine 17 years ago... At the time it was too soon for me to understand anything technical on antennas but the dimensions to build them...
Best 73
Simone - IU1AFY/F4VPY
2013/7/17 i8cvs [email protected]
Hi Zilvinas, LY2SS
I agree completely with you because the QFH was used for the beacon of OSCAR-7 at 2304 MHz and in addition to be endfire radiation or bakfire radiation it do not require a reflector.
The size of a QFH for 70 cm is very small as you quoted in 264 mm height only and 84 mm in diameter and so it is not very big and weight not to much for a balloon particularly because there is not a reflector in it.
I hope that you as well Phil KA9Q and Simone IU1AFY have received my pdf file concerning the QFH written by Walter Maxwell W2DU and another one for 435 MHz written by my self in italian for Radio Rivista 10/1998
I suggest you to read also my article:
"Experimental investigation of Quadrifilar Helix Antennas" published by the AMSAT Journal May/June 2004
Best 73" de
i8CVS Domenico
----- Original Message ----- From: "Zilvinas, LY2SS" [email protected] Cc: "AMSAT BB" [email protected] Sent: Wednesday, July 17, 2013 12:32 PM Subject: [amsat-bb] Re: Help wanted on CP antenna design
On 2013.07.17 10:11, Simone wrote:
Hi Phil,
QFH is effectively circularly polarized, it is often used for GPS receivers, since when you point it skywards the main lobe points trough the zenith, while nulls are pointing to the horizon (where you do not
have
satellites normally).
In my opinion QFH risks to be quite bulky for a balloon, since it will occupy a cylinder 1 wavelength high (70cm in your case).
QFH for 70cm band is 264 mm height only (and 84 mm in diameter).
73! _______________________________________________ Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite
program!
Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
Phil,
you probably know an order of magnitude better than me, but anyway: if you have the freedom to design the transmission mode, have a look at the techniques used by DVB-T/DVB-H, which face similar or worse multipath problems (transmitter is much closer to the ground): use COFDM, splitting the bandwidth into multiple subcarriers, and them modulate each subcarrier using QPSK or even QAM. Then you are making the symbol period much longer, improving immunity against multipath; but at the same time your symbols are much more complex, thus binary efficiency is not much worse than a similar single-carrier system. At high speeds doppler becomes a problem, but it's a matter of achieving a balance between multipath immunity and doppler tolerance.
Miguel EA1ICZ
2013/7/17 Phil Karn [email protected]
I'm one of several adult mentors to a local high school ham club that's been designing and flying balloons. We are designing a high speed 70cm digital downlink for a future payload. One of my major concerns is frequency selective multipath fading in the 1 MHz bandwidth I plan to use as the balloon gets low on the horizon of the tracking station and the receiver begins to pick up a reflection off the ground.
I think one of the simpler ways to deal with multipath is to use circular polarization on both ends. This automatically rejects all odd-order reflections (including the all-important 1st order reflection off the ground) as they have the opposite circularity sense to the direct signal and automatically be cancelled at the receiver. (CP was tried experimentally with analog TV broadcasting back in the 1970s as a means of reducing ghosting.)
On the ground we'll just use a regular 70cm satellite antenna, but on the balloon I need a 70cm circularly polarized transmit antenna with a fairly wide (hemispherical) beamwidth that can maintain its circularity over as much of that as possible.
I'm not an antenna guy, but my understanding tells me that a quadrifilar might be a good choice here. Any comments on this? Can anybody point me to some typical performance figures for these antennas? Again, I'm more interested in cross-polarization rejection than in absolute gain. I have plenty of link margin and I'm willing to sacrifice a few dB if necessary to get rid of those deep multipath fades.
Thanks,
Phil
Sent via [email protected]. Opinions expressed are those of the author. Not an AMSAT-NA member? Join now to support the amateur satellite program! Subscription settings: http://amsat.org/mailman/listinfo/amsat-bb
participants (6)
-
i8cvs -
Miguel Barreiro -
Phil Karn -
Simone -
Zach Leffke -
Zilvinas, LY2SS