Today's Message Index:
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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
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| Subject: | Re: IS a COM ANTENNA GROUND PLANE NECESSARY |
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
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| Subject: | 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
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| Subject: | 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
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| Subject: | 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 . . .
Bob . . .
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