70 cm Phased-Array Antenna - Elmers Forum at eHam.net

$\'\'$

$\'\'$

$\'\'$

eHam.net Forum : Elmers : 70 cm Phased-Array Antenna

Forum Help

1-5 of 5 messages

Page 1 of 1

70 cm Phased-Array Antenna	Reply
by W4PRT on July 17, 2002	Mail this to a friend!
Howdy, Due to circumstances beyond my control, I am having to move back to a place that has antenna restrictions. So, I thought that I would build an antenna that I could erect when and if I choose. I read an article on the ARRL site that showed how to build a 2-Meter Phased-Array Antenna. So, I want to build a 70cm version of the same antenna. The original article is at: http://www.arrl.org/members-only/tis/info/pdf/9801061.pdf Can anyone help me make this antenna based on the original article? Thanks, Pete (W4PRT)

RE: 70 cm Phased-Array Antenna	Reply
by WB2WIK on July 17, 2002	Mail this to a friend!
That's only a pair of phased 1/4-wave ground planes with 90 degrees phase stagger and 1/4-wavelength separation to achieve the standard cardioid pattern. For 70cm, assuming a center frequency of 446.0 MHz, the 1/4-wave ground plane could be 6-1/4" vertical radiator with 6-5/8" long radials, separated by 6-1/4" (side to side). Scale the RG8X phasing lines by 1/3, and you're done. On 446 MHz, I'd recommend going to BNC or type N connectors instead of the UHFs shown. The RG8X can still be used, provided it's high quality stuff. I would not expect great performance from such a system, and if you're building a directive antenna which will require rotation anyway, would recommend a 5-6 element Quagi instead. Far more bang for the buck, and still a very tiny, unobtrusive antenna. 70cm Quagi instructions are available at http://www.n6nb.com. WB2WIK/6

RE: 70 cm Phased-Array Antenna	Reply
by W4PRT on July 18, 2002	Mail this to a friend!
Thanks for the reply, faster than I thought that I would receive it...;-) You wrote "For 70cm, assuming a center frequency of 446.0 MHz, the 1/4-wave ground plane could be 6-1/4" vertical radiator with 6-5/8" long radials, separated by 6-1/4" (side toside). Scale the RG8X phasing lines by 1/3, and you're done." Please bear with me, I am new at building any kind of antenna and want to get this right. I am assuming that the 6-1/4 side to side is for the 1/4 wave space of the PVC that will hold the groundplanes, but what are the lengths of the coax for the "phasing harness". I am a bit confused when you say "scale the lines by 1/3", do you mean reduce the lengths in the article I refrenced by 1/3? Thus making one length, 7-1/3 and the other 5-1/3? As for performance, I am not looking for several hundred miles per contact,but better than a duck and a learning experience at building an antenna. What kind of range do you think that I will get out of this setup, say at 300mW 5 watts and 35 watts respectively? Thanks again, Pete (W4PRT)

RE: 70 cm Phased-Array Antenna	Reply
by K5DVW on July 18, 2002	Mail this to a friend!
Hi Pete, Building antennas is real fun, as you've started to learn. I think when Steve said to scale the coax phasing lines by 1/3, that is to say multiply the given 2m lengths by approximately 1/3. This should give the same phase relationship since 144 Mhz is about 1/3 of 440 Mhz. If you want to be exact, then scale them by 146/445 = 0.32, but that's so close to 1/3 as you see. As for the distance you'll get, are you talking about repeaters or simplex, or FM or SSB? When talking about UHF distances, antenna height above ground is the single biggest advantage, then comes power/antenna gain. You'll be operating line of sight, so once you have established that, then getting enough power to make a decent contact will be next. But, if you dont have line of sight, it wont work! The line of sight equation is distance in miles = (antenna h in feet *2)^0.5 or in better terms, the square root of 2 times the antenna heigth in ft. This is a common formula for radio horizon. This is just for one end of the radio link, for both ends say from your house to a repeater, do the math and add the miles. Sorry for the indirect answer, but 300 mW could get you only down the block, or 100 miles depending on the situation. Have fun K5DVW

RE: 70 cm Phased-Array Antenna	Reply
by WB2WIK on July 18, 2002	Mail this to a friend!
Yep. Unfortunately (well, maybe fortunately for us hams who love to fool around and experiment with things!), there is no sure model for radio contact prediction because there are too many variables and unknowns. What you can "work" with your radio and antenna is largely decided by what the _other_ station is using, not only what you are. Also, antenna height above earth, height above average terrain, your transmission line loss, receiver sensitivity -- as well as all those parameters for the "other" station who you're trying to contact -- must be known. And the real path loss is also determined by what is contained in the actual path. Trees? Utility lines? Buildings? Any terrain (hills) in the path? Even the relative humidity on the day you're trying to make contact has an effect on 70cm propagation. I recommended the Quagi, rather than two phased ground planes, because construction cost is nearly identical (and nearly zero, in either case!), work expended is similar, and results will be far better. The phased ground planes could achieve about 3-4 dB gain, while a small Quagi weighing less than one pound can achieve 10-12 dB gain. I'd opt for the higher-gain system. WB2WIK/6

Page 1 of 1

Next Topic: ICOM IC-706 intermod problems on hi gain hf antenn
Previous Topic: is it tore up???

Email Subscription
You are not subscribed to this topic.

Check our help page for help using Forum, or send questions, comments, or suggestions to the Forum Manager.

$\'\'$