---------------------------------------------------------- AeroElectric-List Digest Archive --- Total Messages Posted Mon 08/03/20: 4 ---------------------------------------------------------- Today's Message Index: ---------------------- 1. 05:27 AM - Re: IS a COM ANTENNA GROUND PLANE NECESSARY (user9253) 2. 11:05 AM - Re: Re: IS a COM ANTENNA GROUND PLANE NECESSARY (Robert L. Nuckolls, III) 3. 11:07 AM - Re: Re: IS a COM ANTENNA GROUND PLANE NECESSARY (Robert L. Nuckolls, III) 4. 05:21 PM - Re: SWR minima not at resonance? (Robert L. Nuckolls, III) ________________________________ Message 1 _____________________________________ Time: 05:27:58 AM PST US Subject: AeroElectric-List: Re: IS a COM ANTENNA GROUND PLANE NECESSARY From: "user9253" My friend abandoned the antenna embedded in the vertical stabilizer on his Kitfox. He purchased a new antenna and mounted it atop the fuselage just aft of the cargo area. The antenna base is mounted to a steel plate that is part of the tubular steel airframe. The SWR was 1.95 when transmitting on 122.75. The ground plane could probably be improved if necessary. The wings were folded back during the SWR test. The aluminum flaperons were within 6 inches of the antenna. Question: Do nearby metal objects (flaperons) affect the SWR? If so, the the SWR test could be repeated when the wings are unfolded. -------- Joe Gores Read this topic online here: http://forums.matronics.com/viewtopic.php?p=497489#497489 ________________________________ Message 2 _____________________________________ Time: 11:05:36 AM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: Re: IS a COM ANTENNA GROUND PLANE NECESSARY At 07:26 AM 8/3/2020, you wrote: > >My friend abandoned the antenna embedded in the vertical stabilizer on his >Kitfox. He purchased a new antenna and mounted it atop the fuselage just aft >of the cargo area. The antenna base is mounted to a steel plate that is part >of the tubular steel airframe. Good >The SWR was 1.95 when transmitting on 122.75. You need to run a spectrum plot. Check SWR every 0.5 Mhz from 118 to 132 and plot a curve. >The ground plane could probably be improved if necessary. May not be necessary . . . you won't know for sure until you get the 'big picture' Generally speaking, an narrow-band antenna can have a perfect or even low SWR at only one frequency. That 'dip' in SWR may or may not be at the antenna's resonant frequency. The attached plot illustrates an exemplar antenna's performance over a range of frequencies. The antenna's 'sweet spot' is at 429Mhz. The antenna is satisfactory (2:1 or less) over a range of about 416-442Mhz. Useful (3:1 or less) over a range of 403-452Mhz. Your SWR meter is a minimal utility vector network analyzer. It just doesn't 'scan' or 'plot' for you like a full featured VNA. However, with a little data gathering and pencil work on a graph, you can still get the information you need. >The wings were folded back during the SWR test. The aluminum flaperons were >within 6 inches of the antenna. Question: Do nearby metal objects >(flaperons) affect the SWR? If so, the the SWR test could be repeated when >the wings are unfolded. You want to do testing with the aircraft as close to a flight condition as practical. Yeah, conductors in close proximity do have an effect. Bob . . . ________________________________ Message 3 _____________________________________ Time: 11:07:34 AM PST US From: "Robert L. Nuckolls, III" Subject: Re: AeroElectric-List: Re: IS a COM ANTENNA GROUND PLANE NECESSARY >The wings were folded back during the SWR test. The aluminum flaperons were >within 6 inches of the antenna. Question: Do nearby metal objects >(flaperons) affect the SWR? If so, the the SWR test could be repeated when >the wings are unfolded. You want to do testing with the aircraft as close to a flight condition as practical. Yeah, conductors in close proximity do have an effect. Just for grins, you might run a scan with the wings folded then repeat the exercise with the aircraft in flight configuration. I would be interesting to see how much effect there is due to proximity of folded wings. Bob . . . ________________________________ Message 4 _____________________________________ Time: 05:21:33 PM PST US From: "Robert L. Nuckolls, III" Subject: AeroElectric-List: Re: SWR minima not at resonance? Followup: Earlier I wrote: >Generally speaking, an narrow-band antenna can have >a perfect or even low SWR at only one frequency. >That 'dip' in SWR may or may not be at the >antenna's resonant frequency. A Lister wrote that he thought lowest swr was the primary manifestation resonance. I told him I would elaborate with a posting to the List. Here's an exemplar plot of characteristics for one of several communications antennas I evaluated for use on our local EMS/ LE/FIRE services: Note that the SWR minima is at 493.6Mhz while the reactive component swing (resonance) is at 457Mhz. At lowest SWR, the impedance of the antenna is 56.9 Ohms, resistance is 56.6 Ohms, reactance is 5.8 Ohms inductive. The 56.9 Ohm impedance produces the not quite perfect but still satisfactory value of 1.18:1. One COULD craft a matching network that would yield 1:1 somewhere . . . but it would still rise either side of optimal. In the illustrated case, the 1.5:1 bandwidth (markers 1 and 2) is 66Mhz. NOT centered on our frequencies of interest in the 460 to 470Mhz but still quite satisfactory for task at hand with SWR of much less than 2:1 over the range. This is the kind of data you can get from a Vector Network Analyzer, a test tool discussed here on the List a few weeks ago. Quite illuminating when it comes to matters of feedlines and antennas. The antenna Joe is working with will hopefully offer 2:1 or better over his range of interest, 118 to 132Mhz. Bob . . . 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