Helix antennas can be constructed to provide usefull gain with up to 40 turns if designed with a circumference of one wavelength and a pitch angle of 12.5 degrees. In my direct experimental comparison measurements outlined in the May/June 2006 AMSAT Journal the max gain formulas used on various web sites and amateur publications, for as yet some undetermined reason, do not provide the predicted gain, but had less gain and increased side lobes. The longer the antenna the greater the deviation from that predicted.
As stated previously feed line losses are a big problem for many hams at 1.2 GHz. My array (VE3NPC) is fed with 5 ft of RG/8U in the shack, 35 feet of 1/2 in hard line and another 5 feet of flexible RG/8U at the antenna.
I have 10 watts from my FT-736 and have never measured what it is at the antenna. It perfomed very well on AO-40 where I logged 832 SSB QSO's in mode L/S.
For helix design dimensions go to http://www.rac.ca and select calculators, helix antennas and then Kraus. (not Emerson as those max gain dimensions result in a poorly performing antenna).
Clare VE3NPC
----- Original Message ----- From: "kc6uqh" kc6uqh@cox.net To: w7lrd@comcast.net; "Steve Raas" sraas@optonline.net; amsat-bb@amsat.org Sent: Tuesday, May 08, 2007 12:46 AM Subject: [amsat-bb] Re: Helix vs. loopers
You loose 3 dB linear to circular, circular to circular of the same rotation is best for satellite work eliminating fading during rotational shift and multipath. I agree about diminishing returns on a Helix. I have phased two for linear polorization ( counter rotating 1.2 GHz). The BBQ grill is a simple solution, 18 dBi 1.2 and 24 dBi 2.4 with a small form factor, I use it portable and it fits in my car. If you have room for a 12' boom why not a 10' TVRO dish? You could go portable with a Semi Truck!
Art ----- Original Message ----- From: w7lrd@comcast.net To: "Steve Raas" sraas@optonline.net; amsat-bb@amsat.org Sent: Monday, May 07, 2007 9:13 PM Subject: [amsat-bb] Re: Helix vs. loopers
Steve and all I believe simply lengthing a helix will reach a point of diminishing returns. It would be better to stack a couple or even four twenty or so turn turn helix antennas (from VE3NPC). You could with proper phasing generate a CP signal from a couple of loopers. I just haven't heard of anyone doing that, though it looks good on paper. 73 Bob W7LRD Seattle
-------------- Original message -------------- From: Steve Raas sraas@optonline.net
In reading much of the discussion that has hit the BB lately in regards to antennas I figured Ide toss in my questions to the group.
In trying to figure out which style antenna is going to suit my needs best I have a few questions.
#1 Is a linear polarized antenna less efficient than a RHCP antenna when trying to receive or transmit to a rhcp signal / receiver?
#2 If yes.. in theory how much?
#3 In looking @ pre-built helix options for 1.2g I have found the DSH23-12 which is a 23 cm RHCP 12 turn helix 14.5 dBic gain in a machined radome for best weather protection. 1.2-1.3 GHz bandwidth. 50 ohm feed wit h N (f) connector on 8" backplane. All stainless hardware; Designed for PHASE 3D uplink operation. Wide bandwidth actually covers from 900 - 1600 MHz ( copied verbatium from the www.directivesystems.com website. In looking at the specs of this helix, it seems it may not have the desired gain for P3E or others. In 'theroy' is it possible to make a 24 turn helix , and if so would I be correct in thinking that double the turns would provide at or near 3.0 db of additional gain?
#4 If yes to question 3 would this antenna be @ or near double the physical length?
In a email I sent to Dave @ Directive he told me that the loopers he has can be arranged for vert/linear or horizontal polarization, so I do think that a looper although a great antenna may not be the best single antenna solution for 1.2 Uplin k.
Thoughts, Ideas & comments please.
-Steve Raas