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Author Topic: Homebrew 2M/440 Antenna....any new ideas?  (Read 2943 times)
KB9YKG
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Posts: 8


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« on: October 16, 2001, 04:09:05 PM »

I am getting ready to dive into another small antenna project, and I am hoping that some of you have some suggestions.  The antenna is for the 2m/440 ham bands.  I am using an Icom IC820, and Belden 9913 feedline.  Here are a few qualities that the antenna needs to have:<br><br>
1.  Small in size, and fairly lightweight.<br>
2.  Vertical polarization, so I can access the local vhf/uhf repeaters.  Most of the repeaters are nearby, so a high gain design is not really needed.<br>
3.  I would like to be able to work UO-14, and AO-27 if possible.  I am willing to give up some antenna gain so that more signal can be radiated upwards to achieve this.<br><br>
A big limiting factor for me is living in an apartment.  I have a balcony on the third floor of my complex, but I can't put up any large antennas there.  That is why the design needs to be somewhat small, and lightweight.  If anyone knows of a design that might work for me, please let me know, or point me toward a websight that may help.  Thanks.
<br><br>
73's
<br>
Andrew Knepler
<br>
KB9YKG
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KB9YKG
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Posts: 8


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« Reply #1 on: October 16, 2001, 04:11:52 PM »

Well, I guess no html is supported.  My mistake...sorry....
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WB2WIK
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Posts: 20543




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« Reply #2 on: October 17, 2001, 10:53:11 AM »

Unfortunately, regardless how much space you have, using an antenna that's effective for FM/repeater work and satellite work all rolled into one system is difficult at best; I consider it nearly impossible.  For FM/repeater work, you obviously want something that's vertically polarized and has the lowest possible radiation angle, to reach the farthest.  It doesn't matter if that antenna is blocked completely towards the sky, since you're not working the sky; you want it to be completely clear of obstructions to the sides, and preferably, to all sides.

For satellite work, the goal is obviously quite different!

I've found the M-squared "Eggbeater" antennas to work satellites reasonably effectively, especially if they are used in conjunction with a zero-loss feedline (put the rig right at the antenna), or with a remote power amplifier for TX and remote receive preamplifier for RX, to counter the effects of cable loss between the antennas and the rig.  They also work FM/repeaters pretty effectively, since they do create substantial radiation at relatively low angles as well.  They have no real "gain," they just seem to work, and work quite well for their very small size.

Of course, the "ultimate" in your situation would probably be to go to circularly polarized beam antennas mounted on an elevatable cross-boom held with a clamping device to maintain elevation and azmuth optimally for your intended path.  There are small ones that can be built, using crossed linear elements and a phase shift network at the feedpoints, or they can be purchased from M-squared and others.  The smaller ones for 144 and 432 MHz are not large and would fit on a balcony, supported by a tripod and el/az mechanical mount that really requires no motorized rotation system if you can simply reach the antennas and aim them.

Recommend you take a browse through the M-squared website for a start.  But many smaller companies, including some European ones, offer excellent products that might be helpful.  The AMSAT website has some very good information and links.

73 de Steve WB2WIK/6
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WB6BYU
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Posts: 13027




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« Reply #3 on: October 17, 2001, 11:24:36 AM »

If you don't need a lot of gain, then a simple 2m quarter wave
ground plane (or J-pole or vertical dipole) should work for the
repeaters on both 2m and 70cm.  You can build a simple ground
plane by soldering pieces of wire to a female chassis-mount coax
connector, and it is far cheaper but just as efficient as one
purchased commercially.

I'd recommend you use two antennas:  one for satellites (probably
a turnstyle or eggbeater on the balcony, which may require one
for each band) and a separate one for repeaters.  You may be
able to mount the latter indoors by the rig if that gives you good
enough coverage to the repeaters you want.

If you are interested in satellites, then install a good antenna
system designed for that.  You may find that there is enough
incidental radiation to work the repeaters.  If not, build a simple
ground plane for that purpose.
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KE4DRN
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Posts: 3714




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« Reply #4 on: October 17, 2001, 08:51:40 PM »

Hi,

ARRL QST Aug 2001  page 38

w4rnl has construction article for a 2m and 70cm
turnstile antenna using Moxon design.

Dr. Cebik has done it again with another great
project and easy to understand text and clear photos.

73 james
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KE4DRN
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Posts: 3714




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« Reply #5 on: October 17, 2001, 08:56:26 PM »

Hi,

For 2m repeater and simplex use try this one:

http://www.cebik.com/hs.html

The half square antenna is easy to build and for a few
dollars you are on the air !

Easy to put up in a room or outside and great results.

73 james
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W9GB
Member

Posts: 2599




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« Reply #6 on: November 07, 2001, 01:50:04 AM »

You should consider the "Arrow antenna"

http://hometown.aol.com/Arrow146/index.html
http://hometown.aol.com/Arrow146/146-437.html

eHam.net reviews for the Arrow can be found at:
http://www.eham.net/reviews/detail/1369

The only potentail "disadvantage" is that it is a hand held antenna (operator or tripod required).

Both AO-27 and UO-14 are easier to use with a directional antenna and a hand held antenna is "much" cheaper, than a az-el system to track the sky for these "short" (less than 15 minute) passes.

You can also use the antenna for ISS packet and voice work - especially when ISS gets their new antennas in 2002 !

I also like the new "packng case" for mobile or field day operations with a handheld or small transceiver.
 
Greg
w9gb
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K0RFD
Member

Posts: 1368




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« Reply #7 on: October 25, 2003, 11:29:50 AM »

For an easily homebrewed dual-band base antenna, it's hard to beat the all-aluminum Arrow J-pole.  The design is on the web.  Allan Lowe from Arrow has put the plans up at:

http://www.krell.com/amateur/antenna/jpole/

But I wouldn't use it for satellites. There are better alternatives.

Let me correct some commonly quoted misinformation about working the "easy" sats.  You really don't want to get your signal going *up* to work the Low Earth Orbit (LEO) satellites.  The main reason is path loss.  You need quite a few dB more to hear a satellite when it is near the horizon, because the signal is passing thru thousands of miles of atmosphere.  When the bird is near the zenith, you get as much as 12-15 dB more signal.

Now couple that with the concept (easily forgotten) that a LEO satellite is RARELY near the zenith anyway.  When the bird is near the horizon, its apparent motion relative to your position is quite slow (because it is moving mostly toward you or away from you); when it is near the zenith, the apparent motion is quite fast (because it is moving PAST you at 17,500 miles/hr).

I modeled two full weeks of six-a-day UO-14 passes at my QTH by using a tracking program and sampling the satellite track every 15 seconds.  I found that the average satellite elevation here during a pass was 14 degrees above the horizon.  That includes the passes where it goes straight overhead, but as I said, its apparent motion is quite fast at that time.

That's pretty much what's wrong with W4RNL's Moxon turnstile idea.  I built one of these, and it wasn't a particularly outstanding performer.  Why? because it looks mostly up at high angles, when you really need the signal at *low* angles.  Dr. Cebik is a brilliant man and has some GREAT information on his site--when building my station I couldn't have survived without him--but the concept of using a Moxon turnstile design for satellites was a little off.  If you have to use a fixed antenna, K5OE's Eggbeater II design is more appropriate (lower radiation angle) and just as easy to build.

I agree with the idea that an az-el system is the best, however there are alternatives.  I earned my satellite VUCC on the FM satellites using an HT for 440 receive and a 2m mobile rig for transmit.  I built a little handheld 4-element yagi for the downlink antenna (mounted a Ramsey preamp and 9 volt battery right in the handle), and used the 1/4 wave whip on the car for my uplink.  A mobile 1/4 wave whip, believe it or not, sends signal at just about the right angle for the LEO satellites.  Most of the time I could hit the birds with the mobile set to only 5 watts; using 2 radios gave me full duplex.  If you don't feel like homebrewing, it's pretty hard to beat the Arrow handheld antenna.  It's pretty much the standard for the FM birds. Handheld antennas are great.  When the bird changes polarization on you, you just flip-flop the antenna.

If you have some extra bucks, find a used azimuth-only TV rotor and mount a couple of yagis pointed at a fixed elevation angle.  That's what I did when I finally graduated from the HT/Mobile setup.  For downlink, I built a circular polarized 440 yagi that had left-hand and right-hand switched with a relay, and just used a vertically polarized 2m yagi for uplink.  (remember, the receivers on the satellites are GREAT--they can hear very weak signals; it's the transmitters, typically less than a watt, that challenge you).  That's why I did the pass-elevation modeling I mentioned above.  A modest yagi is going to have a enough -3dB beamwidth to get you nearly to the zenith if you point it a little above the horizon, and even if the signal is down more than 3dB, you really don't need it when the satellite is that high.  I pointed mine at 8 degrees.  Even though the average pass was 14 degrees, I felt I needed more gain for the lower portions of the pass.  It worked fine that way.  And the 2meter yagi was killer into the repeaters if you pointed it at them.  I could routinely get into a repeater 120 miles away with it, even though it was pointed up a little.

And DO use a mast-mounted preamp in line with your downlink antenna.  Even the little $12 Ramsey 440 preamp (kit built) works great.  By mounting it up on the mast, it will amplify the received signal at the antenna where it is the strongest, and will overcome quite a bit of feedline loss.  You can't transmit thru the inexpensive Ramsey (it has no RF sense switching), but if you're using 2 rigs that's not a problem.  If you're using 1 rig and 2 antennas for uplink and downlink, you'll need a duplexer in line to keep from frying the preamp.  But you'll need that anyway.

I wish I had a nickel for every guy I heard on the FM birds that had a great signal INTO the satellite but couldn't hear at all when you came back to him.  An inexpensive preamp solves that problem nicely.
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