Antenna questions

[Steve Ruse]

I recently purchased a Vertex Standard VXA-150 handheld radio for use in my
(wood frame) Pietenpol. I am now trying to determine what antenna would be
best to extend my range. Would likely see a substantial difference in range
between a ground plane antenna dipole antenna such as the Advanced Aircraft
Electronics 5T antenna?

I'm also really interested in building my own antenna. I need to do some
reading on this, but wouldn't mind having the opinion of a few of you guys.
I am an Electrical Engineer, but antennas are far from being my strong
point. How difficult would it be to construct my own dipol antenna? Is it
as simple as running coax to two conductors of the proper length placed end
to end? I do intend to do some of my own research on this, but I would like
to know what kind of real-world results some people are getting. If anyone
wants to point me towards any good sources it would be much appreciated.

Thanks!

Steve Ruse
Dallas, TX

[Ron Wanttaja]

On Wed, 12 Jan 2005 05:47:26 GMT, "Steve Ruse" > wrote:

>I recently purchased a Vertex Standard VXA-150 handheld radio for use in my
>(wood frame) Pietenpol. I am now trying to determine what antenna would be
>best to extend my range. Would likely see a substantial difference in range
>between a ground plane antenna dipole antenna such as the Advanced Aircraft
>Electronics 5T antenna?
>
>I'm also really interested in building my own antenna. I need to do some
>reading on this, but wouldn't mind having the opinion of a few of you guys.
>I am an Electrical Engineer, but antennas are far from being my strong
>point. How difficult would it be to construct my own dipol antenna? Is it
>as simple as running coax to two conductors of the proper length placed end
>to end? I do intend to do some of my own research on this, but I would like
>to know what kind of real-world results some people are getting. If anyone
>wants to point me towards any good sources it would be much appreciated.

You might take a look at:

http://www.bowersflybaby.com/stories/antenna.htm

It's a conventional antenna made from Home Depot and Radio Shack parts. Been
working fine for three years.

Ron Wanttaja

[ELIPPSE]

Hi, Steve!
I made all of the antennas for my Lancair; com, VOR, glide-slope, and
transponder. You can make a simple 1/2 wave com or VOR dipole out of
sheet aluminum shaped like a bow-tie. The experimental ones I made had
a VSWR of less than 1.2:1 over the full 108-136 range. By making them
very wide, their electrical length can be much reduced. You can feed it
either into the center using a clamp-on ferrite over the feed-end of
the coax to act as a balun, or feed in from one element-end toward the
center. In this mode you keep the coax up against the antenna, but
insulated from it, and cover the coax the length of the element with a
length of aluminum formed in a "V" cross-section. This serves as a
"bazooka" balun. The balun is very critical in both maintaining
BALanced-to-UNbalanced conversion, and to keep antenna currents from
flowing down the coax back toward the radio. These currents can often
get into the microphone feed and cause squeeling and distortion during
transmit.
The 1/2 wave dipole is a real performance increaser over the 1/4 wave
dipole over a counterpoise, so-called ground-plane. We generally have
insufficient width of the counterpoise to maintain good radiation
patterns down to horizontal angles. With this deficiency, along with
mounting an antenna on upper rather than lower surfaces where the major
radiation is upward, keeps us from having good long-range
communications. You are fortunate in having non-conductive construction
which allows you to place an antenna inside the structure. The com is
vertical polarization, and if you still use VOR, that is horizontal
polarization. Be sure to keep the antenna as far from vertical (com) or
horizontal (VOR) metallic surfaces such as tubes, cables, wires, and
struts as these can contribute re-radition which will form lobes in the
pattern with resulting signal nulls at different aspect angles. Make
each element about 15" long, tapering from 1/2" wide at the feed to
about 12" wide at the end. They can be either triangular or
sector-shaped, in other words straight or curved ends. Separate the
feed ends by about 3/8". Attach the shield to one feed end and the
center conductor to the other. The material I used was 0.010" aluminum
flashing obtained from the local hardware store. Simple, huh?

[GeorgeB]

On 13 Jan 2005 10:38:09 -0800, "ELIPPSE" > wrote:


>I made all of the antennas for my Lancair; com, VOR, glide-slope, and
>transponder. You can make a simple 1/2 wave com or VOR dipole out of
>sheet aluminum shaped like a bow-tie. The experimental ones I made had
>a VSWR of less than 1.2:1 over the full 108-136 range.

ELLIPSE, I am impressed. I've played with RF for some 45 years, and
have a hard time getting a true 1.2:1 VSWR into a resistive dummy
load, much less into an antenna over a bandwidth exceeding 20%. I'd
like to get with you and see what we can do in getting a patent on
your invention; you will be a wealthy man.

By any chance have you also done any radiation patterns on this?
Since it is vertical, I suppose it is perfectly circular, but I wonder
about the bank angles; does it work well when you are in steep banks?
From your skills at attaining that VSWR, I bet you get spherical
radiation with a 5 or 6 dB gain.

>These currents can often
>get into the microphone feed and cause squeeling and distortion during
>transmit.

So you have had some problems with VHF signals conducted on the
feedline getting into the microphone circuit and causing audio
feedback?

>Make
>each element about 15" long, tapering from 1/2" wide at the feed to
>about 12" wide at the end. They can be either triangular or
>sector-shaped, in other words straight or curved ends. Separate the
>feed ends by about 3/8". Attach the shield to one feed end and the
>center conductor to the other.

Hm, I thought the driving point impedence of a 1/2 wave dipole was
substantially different from 50 ohms ... your balun handles the
balanced to unbalanced conversion; what does the impedence
transformation?

Do you have a webiste detailing this design, especially with
supporting measurement information? It has many applications far from
aviation, as well as in aviation.

George

[ELIPPSE]

Hi, George!
Brown and Woodward did the tests in 1945, published in 1952, of the
impedance and patterns of conical and triangular antennas, both against
a ground plane and also biconical and triangular bow-tie dipole
antennas. Depending upon the length, impedance values ranging from much
less than 50 ohms to as much as 270 ohms were obtained. Their 120 deg.
flare biconical demonstrated a VSWR<2 over a 6:1 bandwidth with a cone
diameter D=lambda at the lowest frequency.
I've made numerous VSWR tests of triangular dipoles of various flare
angles, trimming the length to obtain minimum VSWR at the mid-point of
the frequency band of interest. Mid-band VSWR was often in the 1.05 to
1.10 region. I've never done pattern measurements of these antennas,
but I rely on B&W's pattern measurements which show maximum radiation
normal to the line of the dipole. The gains they show are within +0.5dB
to -0.5dB relative to a 1/2 wave dipole for the shorter antennas.* I've
performed numerous pattern measurements of anything from 200 MHz Yagi
arrays to x-band horn, conical-scan, pill-box and large diameter
parabolic arrays. I've also done much testing of multipath reduction
fences to reduce pattern-interfering ground reflections into tracking
antennas at Vandenberg AFB.
*See "Antennas, John D. Kraus, 2nd Ed. pps. 354-358 Paul

[ELIPPSE]

Hi, George!
Addendum: Yes, I have helped several people who had squealing in their
com radios at several transmit frequencies. In two of the cases, using
the clamp-on ferrite toroid-halfs with 1/4" ID and about 1" length,
placed over the coax as close as possible to the antenna took care of
it. It would even suppress the antenna currents as long as it was
between the antenna and where the radiation was getting into the mic.
lead. In several of the other cases, close proximity of leaky RG58
with the microphone and or PTT cable introduced the cross-talk. By the
way, what we call the PTT, Push-To-Talk, the Brits, cheeky buggers that
they are, refer to it as PTS, Press-To-Speak!
I'll soon have a web-site; my daughter is setting one up for me. If
there is an interest, I'll show the triangular dipole antenna
dimensions and graphs of VSWR vs. frequency. They can be scaled for
other frequencies! The dimensions were not all that critical; trimming
of the length mainly was to get the VSWR symmetrical about the band
center. Even at extreme frequencies the VSWR was quite low!
The antenna I made for the transponder initially made me fail my
transponder test by pulling the transmit frequency too far off. It was
a slot-fed dipole as illustrated in the Rad Lab Series, Vol 12,
Microwave Antenna Theory and Design, Silver, 1949, p.246, Fig. 8.5. The
King KT -76A has its output coming from a tuned stage, and is very
sensitive to frequency pulling from the reactive components of standing
waves; they prefer a tuned transmission line. What I did was insert
various lengths of BNC adapters - barrels, M-F, right angle, etc. until
I got the reflection's reactive component to the correct value to get
the frequency correct! Paul

[Blueskies]

"ELIPPSE" > wrote in message ups.com...
> Hi, George!
> ...
> I'll soon have a web-site; my daughter is setting one up for me. If
> there is an interest, I'll show the triangular dipole antenna
> dimensions and graphs of VSWR vs. frequency. They can be scaled for
> other frequencies! The dimensions were not all that critical; trimming
> of the length mainly was to get the VSWR symmetrical about the band
> center. Even at extreme frequencies the VSWR was quite low!
> ...>

Looking forward to seeing your web site....

[ELIPPSE]

Hi, George!
I finally found my notebook with my antenna VSWR results, and sad to
say, I must apologize for an exaggeration about the antenna being less
than 1.2 from 108 to 136; it wasn't. Here's the actual: <1.2 - 119/144;
,<1.5 - 109/148; <2.0 - 108/>150. Here are the dimensions: Each
antenna-half element was shaped as a triangular segment of a circle
15.125" long (radius), 13" across the tips of the segment
circumference, with the two elements separated by 3/4". The inner ends
were 3/4" wide. Increasing the radius to about 16" would probably
center the response over the aircraft VHF band better. Paul

[RST Engineering]

And you measured this in a 50 ohm system? You are in line for a Nobel
Prize, my friend.

Jim



"ELIPPSE" > wrote in message
oups.com...
> Hi, George!
> I finally found my notebook with my antenna VSWR results, and sad to
> say, I must apologize for an exaggeration about the antenna being less
> than 1.2 from 108 to 136; it wasn't. Here's the actual: <1.2 - 119/144;
> ,<1.5 - 109/148; <2.0 - 108/>150. Here are the dimensions: Each
> antenna-half element was shaped as a triangular segment of a circle
> 15.125" long (radius), 13" across the tips of the segment
> circumference, with the two elements separated by 3/4". The inner ends
> were 3/4" wide. Increasing the radius to about 16" would probably
> center the response over the aircraft VHF band better. Paul
>

[jerry wass]

RST Engineering wrote:

> And you measured this in a 50 ohm system? You are in line for a Nobel
> Prize, my friend.
>
> Jim
>
>
>
> "ELIPPSE" > wrote in message
> oups.com...
>
>>Hi, George!
>>I finally found my notebook with my antenna VSWR results, and sad to
>>say, I must apologize for an exaggeration about the antenna being less
>>than 1.2 from 108 to 136; it wasn't. Here's the actual: <1.2 - 119/144;
>>,<1.5 - 109/148; <2.0 - 108/>150. Here are the dimensions: Each
>>antenna-half element was shaped as a triangular segment of a circle
>>15.125" long (radius), 13" across the tips of the segment
>>circumference, with the two elements separated by 3/4". The inner ends
>>were 3/4" wide. Increasing the radius to about 16" would probably
>>center the response over the aircraft VHF band better. Paul
>>

Does this look like one of the old timey TV "Bow Ties " of the 50's ??


>>
>
>

[ELIPPSE]

jerry wass wrote:
> RST Engineering wrote:
>
> > And you measured this in a 50 ohm system? You are in line for a
Nobel
> > Prize, my friend.
> >
> > Jim
> >
> >
> >
> > "ELIPPSE" > wrote in message
> > oups.com...
> >
> >>Hi, George!
> >>I finally found my notebook with my antenna VSWR results, and sad
to
> >>say, I must apologize for an exaggeration about the antenna being
less
> >>than 1.2 from 108 to 136; it wasn't. Here's the actual: <1.2 -
119/144;
> >>,<1.5 - 109/148; <2.0 - 108/>150. Here are the dimensions: Each
> >>antenna-half element was shaped as a triangular segment of a circle
> >>15.125" long (radius), 13" across the tips of the segment
> >>circumference, with the two elements separated by 3/4". The inner
ends
> >>were 3/4" wide. Increasing the radius to about 16" would probably
> >>center the response over the aircraft VHF band better. Paul
> >>
>
> Does this look like one of the old timey TV "Bow Ties " of the 50's
??
>
>
> >>
> >
> >

Hi,
Yes; they look similar to those old UHF bow-tie antennas! Referring to
Fig 8-15 on page 355 of Kraus' "Antennas", it can be seen that the
feedpoint resistance of a 30, 60, or 90 deg. triangular antenna over a
ground-plane is 50 ohms between about 0.17 to 0.2 lambda. Using two
triangular elements each about 0.14 to 0.16 lambda yields 25 ohms each
for a 50 ohm 1/2 wave dipole. Antenna designers have always made use of
thick elements to substantially reduce the drive-point impedance,
physical length and increase the bandwidth of dipoles when practical.
Notice that the 15.125" length of each my antenna's two elements
corresponds to 0.156 lambda, not including feed-point separation. Fig
8-15 also shows that these triangular elements also have near-zero
reactance at these lengths! I'm surprised that the reference I
presented was not consulted before any comments were forthcoming! This
type of antenna is so simple to make that it is unfortunate that those
with airplanes with non-conductive structures have not made use of
their simple construction, low VSWR over a wide bandwidth, and
decreased physical size in making concealed antennas in the past.
Those 1/4" or 1/2" wide copper-tape antennas so many use, with their
constictive resonant bandwidth, high off-resonance VSWR, and long
length would do well to replace them, if possible. If not, one thing
they could do would be to modify them by adding a number of additional
shorter tapes splayed out from the feed-point in a triangular pattern
to approximate the full triangle shape. Perhaps someone could do some
experimentation of this multiple-tape type of antenna and report back
on this forum with the results, so that others could modify their
already-installed antennas this way. Paul