Today's Message Index:
----------------------
1. 06:50 AM - Re: Bob Archer Antenna Voodoo (Charlie England)
2. 07:22 AM - Re: Bob Archer Antenna Voodoo (Bill Boyd)
3. 03:47 PM - Re: Bob Archer Antenna Voodoo (Robert L. Nuckolls, III)
Message 1
| INDEX | Back to Main INDEX |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: Bob Archer Antenna Voodoo |
On Mon, Jul 12, 2021 at 6:28 PM Jeff Luckey <jluckey@pacbell.net> wrote:
> BobN, CharlieE, & RF Gurus et al,
>
>
> I'm considering Bob Archer's wingtip VOR antenna for my RV-7 but I am too
> ignorant to understand how the thing works.
>
> I have read posts all over the intrawebs and have some questions: (Please
> use small words and talk slowly):
>
> 1. It looks to me like the 'front/leading edge' 60 deg leg is bolted
> directly to the base leg which is grounded. How can the "active" (is that
> the correct word?) be directly connected to ground?
>
> 2. I have read articles where the author is talking about "tuning" the
> antenna after it is installed in a specific airframe.
> a. They talk about trimming/cutting pieces - what pieces and how much
> trimming?
> b. They also talk about fooling with the dielectric section but I don't
> understand what physical dimensions/properties they are altering. Are they
> cutting aluminum or bakelite; or moving feed line screw holes; or just
> moving the bakelite piece around in small increments??
>
> 3. SWR meter vs Antenna Analyzer
> a. I think that SWR applies to transmitting, so is it relevant with
> respect to VOR antenna?
> b. What is an Antenna Analyzer and how does it work? (in 5 words or
> less)
>
>
> TIA for VOR Antenna 101
>
>
> -Jeff
>
Hi Jeff,
Anything beyond a plain 1/4 wave or a dipole is beyond me, too. Only thing
I can answer is #1; the center conductor of the coax is only connected 'one
end' of the capacitor formed by the little 2.5" long metal strip, the
bakelite strip, and rest of the '3rd leg' of the triangle. The entire body
of the antenna is *electrically* (at least to a DC ohm meter) tied to the
base strip. So the only thing that the center conductor physically touches
is that 2.5" metal strip, that's DC-isolated from the rest of the antenna.
In the pic, only the two screws with ring terminals are metal. The other
two that tie the 2.5" strip, bakelite, and 3rd leg together are both nylon.
When it comes to tuning, it seems to me that the only practical thing that
mere mortals could reasonably do is trim the length of the outside leg.
The pic shows a purchased Archer Comm antenna; you can see my homebrew nav
behind my hand, complete with green cutting board dielectric. Will it
float? Only Dave knows...
Someone on the interwebs once said that the Archer antenna automagically
turns the entire plane into the antenna. But that's something I read on the
interwebs, so...
Message 2
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: Bob Archer Antenna Voodoo |
Search "gamma match" for more than you want to know on how Bob designed
this antenna. It is DC-grounded, like many antennas. This is
counter-intuitive when you are just immersing your mind into the world of
radiators. "How can it work if it's short-circuited to ground!" At RF, it
does a fine job of coupling energy from the feedline to free space (the
"ether") which is all that matters.
SWR is a concern for Rx as well as Tx because it's all about proper
matching and resulting maximum power transfer from antenna to feedline or
vice-versa. I can tune my "screwdriver" mobile HF antenna by listening for
the sudden increase in received noise as it passes through resonance. That
antenna won't radiate much at all if out of tune, but it's also deaf on the
receive side if not properly tuned. SWR matters for optimum receive
performance. While an out of tune antenna won't blow any finals while
receiving as it can on transmit, it also won't put much signal into the
receiver when you need it to, like using a VOR/ILS.
An antenna analyzer is going to provide much more info than a basic VSWR
meter will, but most of it will be unusable if you don't know what the
values mean. An analyzer is far easier to find resonance with than an SWR
meter (and you'll have to apply transmit power to the antenna to make the
SWR meter read anything) - but an SWR meter and transmit power source are
all the test equip you need for what you're trying to accomplish. Borrow a
nano-vector network analyzer if you can, or drop $60 on one.
-Bill
On Tue, Jul 13, 2021 at 9:55 AM Charlie England <ceengland7@gmail.com>
wrote:
>
>
> On Mon, Jul 12, 2021 at 6:28 PM Jeff Luckey <jluckey@pacbell.net> wrote:
>
>> BobN, CharlieE, & RF Gurus et al,
>>
>>
>> I'm considering Bob Archer's wingtip VOR antenna for my RV-7 but I am too
>> ignorant to understand how the thing works.
>>
>> I have read posts all over the intrawebs and have some questions:
>> (Please use small words and talk slowly):
>>
>> 1. It looks to me like the 'front/leading edge' 60 deg leg is bolted
>> directly to the base leg which is grounded. How can the "active" (is that
>> the correct word?) be directly connected to ground?
>>
>> 2. I have read articles where the author is talking about "tuning" the
>> antenna after it is installed in a specific airframe.
>> a. They talk about trimming/cutting pieces - what pieces and how much
>> trimming?
>> b. They also talk about fooling with the dielectric section but I don't
>> understand what physical dimensions/properties they are altering. Are they
>> cutting aluminum or bakelite; or moving feed line screw holes; or just
>> moving the bakelite piece around in small increments??
>>
>> 3. SWR meter vs Antenna Analyzer
>> a. I think that SWR applies to transmitting, so is it relevant with
>> respect to VOR antenna?
>> b. What is an Antenna Analyzer and how does it work? (in 5 words or
>> less)
>>
>>
>> TIA for VOR Antenna 101
>>
>>
>> -Jeff
>>
> Hi Jeff,
> Anything beyond a plain 1/4 wave or a dipole is beyond me, too. Only thing
> I can answer is #1; the center conductor of the coax is only connected 'one
> end' of the capacitor formed by the little 2.5" long metal strip, the
> bakelite strip, and rest of the '3rd leg' of the triangle. The entire body
> of the antenna is *electrically* (at least to a DC ohm meter) tied to the
> base strip. So the only thing that the center conductor physically touches
> is that 2.5" metal strip, that's DC-isolated from the rest of the antenna.
> In the pic, only the two screws with ring terminals are metal. The other
> two that tie the 2.5" strip, bakelite, and 3rd leg together are both nylon.
> When it comes to tuning, it seems to me that the only practical thing that
> mere mortals could reasonably do is trim the length of the outside leg.
>
> The pic shows a purchased Archer Comm antenna; you can see my homebrew nav
> behind my hand, complete with green cutting board dielectric. Will it
> float? Only Dave knows...
>
> Someone on the interwebs once said that the Archer antenna automagically
> turns the entire plane into the antenna. But that's something I read on the
> interwebs, so...
>
Message 3
| INDEX | Back to Main INDEX |
| PREVIOUS | Skip to PREVIOUS Message |
| NEXT | Skip to NEXT Message |
| LIST | Reply to LIST Regarding this Message |
| SENDER | Reply to SENDER Regarding this Message |
|
| Subject: | Re: Bob Archer Antenna Voodoo |
At 06:23 PM 7/12/2021, you wrote:
>BobN, CharlieE, & RF Gurus et al,
>
>
>I'm considering Bob Archer's wingtip VOR antenna for my RV-7 but I
>am too ignorant to understand how the thing works.
>
>I have read posts all over the intrawebs and have some
>questions: (Please use small words and talk slowly):
>
>1. It looks to me like the 'front/leading edge' 60 deg leg is bolted
>directly to the base leg which is grounded. How can the "active"
>(is that the correct word?) be directly connected to ground?
EASY . . . this antenna is a QUARTER-WAVE RADIATOR working
against a GROUND-PLANE (wing tip root rib) and feed with
a coaxial cable MATCHED to the antenna feed point with
a 'GAMMA MATCH' section. Many successful antenna designs
are 'all metal' wherein it's not obvious how energy
leaving the end of the coax cable can 'radiate' when
attached to a solid hunk of metal that may even be
grounded. It's not magic but it is interesting physics.
For a deeper dive into antenna practice and physics,
here are a few resources you may find interesting.
https://tinyurl.com/rjud4y69
http://www.aeroelectric.com/Reference_Docs/Books/Practical_Antenna_Handbook_4thEd_Carr.pdf
>2. I have read articles where the author is talking about "tuning"
>the antenna after it is installed in a specific airframe.
> a. They talk about trimming/cutting pieces - what pieces and how
> much trimming?
Check out the attached figure. On the right, you see a
depiction of a gamma-fed, 1/4-wave vertical
"grounded" to earth. On the left is an Archer
VOR wing-tip antenna. Both images have principal
performance characteristics labeled.
Dimension 'a' is overall length. This is the 1/4-wave part. It
is adjusted to center the antenna's resonant frequency
in the range of interest. There's a 'calculated'
1/4 wave which is pretty accurate in free space . . .
then there's a 1/4 wave under the influence of
aspect ratio, shape and proximity of surrounding
features.
Dimension 'b' is the distance up from ground that you
attach the gamma-feed strut. Fastening to the bottom
gets you 'zero ohms' . . . moving up increases the
resistance. The magic value for 'b' is that which
gets you the 50 ohm tap to match your feedline.
Dimension 'c' is a mix of things that determine
the value of a capacitor which resonates with the
feed strut to look like a series tuned LC circuit
at the frequency of interest. I.e. it's desirable
that the strut reactance is duplicated by the
capacitor reactance thus making the whole assembly
to appear 'zero length'.
The value of the capacitor will be in a handful
of PICOFARADS, really small capacitor made up
of specifically tailored conductors and a dielectric
(insulator). Successful optimization of your
DIY ARCHER W-T VOR ANTENNA depends on (1)
exact duplication of processes and materials
-OR- (2) testing of your finished product with
any of tools described below to TRIM the dimensions
such that minimum SWR occurs in the center of
the desired spectrum.
> b. They also talk about fooling with the dielectric section but I
> don't understand what physical dimensions/properties they are
> altering. Are they cutting aluminum or bakelite; or moving feed
> line screw holes; or just moving the bakelite piece around in small
> increments??
Generally, space between the conductive parts
separated by the insulating material where
capacity is affected by type of insulation,
thickness of insulation and most importantly,
AREA of the facing conductors.
Amateur Radio antenna builders will make this
capacitor adjustable so that it can be optimized
on the final installation. Bob's 'fixed'
design was optimized in production which CAN
be duplicated DIY but MAY benefit from
analysis after installation with one of
the following tools:
>3. SWR meter vs Antenna Analyzer
> a. I think that SWR applies to transmitting, so is it relevant
> with respect to VOR antenna?
An SWR/Analyzer/VNA evaluation speaks to how well
ENERGY transfers from the physical antenna to
the coax feedline with respect to frequency.
It doesn't matter which way that energy is coming
from or going to. SWR is a universal figure of
merit for one of many qualities of antenna
design and fabrication.
NONE of these tools speak to antenna PERFORMANCE
(gain, pattern, efficiency). You can put a resistor
(dummy load) on the measurement port of any of these tools
and "see" a "perfect" antenna which is in fact,
totally inert. Performance is affected by
antenna shape, materials, fabrication techniques,
installation variables, etc.
So these various analysis tools are most useful
for trouble shooting an antenna system with
a known track record. If crafting a new,
unproven installation, energy transfer is
just a small but important part of the
picture.
> b. What is an Antenna Analyzer and how does it work? (in 5 words or less)
Digital SWR Meter:
https://youtu.be/Egwj5FNsoHc
Analog Antenna Analyzer:
https://youtu.be/jOCG6bFGfuE
VNA:
See https://youtu.be/QJYeFpiqY8c
Any of these devices is useful for evaluating
antenna characteristics. But the VNA gives you
the most data for the least cost.
https://tinyurl.com/yjdo4s6u
>TIA for VOR Antenna 101
>
>
>-Jeff
Bob . . .
Un impeachable logic: George Carlin asked, "If black boxes
survive crashes, why don't they make the whole airplane
out of that stuff?"
Other Matronics Email List Services
These Email List Services are sponsored solely by Matronics and through the generous Contributions of its members.
-- Please support this service by making your Contribution today! --
|