IW5EDI Simone - Ham-Radio

///////////////////////////////////////////
Double Bazooka Coaxial Dipole Antenna

Posted: 14 Apr 2020 04:25 PM PDT
http://www.iw5edi.com/ham-radio/3837...dipole-antenna



Double Bazooka Coaxial Dipole Antenna Info




There is a lot of controversy surrounding the Double Bazooka Coaxial
Dipole. Its bandwidth claims have sometimes been exaggerated.




On this page I give details of how to build two different versions of the
Double Bazooka Antenna. One antenna uses all coax the other uses coax and
twin lead for the antenna.




A DOUBLE BAZOOKA antenna is an extremely broad banded Half Wave Antenna
which can operate efficiently across an entire Ham band with little change
to the SWR. The BAZOOKA antenna design was developed by the staff of M.I.T.
in the early 1940s for use by the U.S. Government as a radar antenna. It
was modified for amateur radio use in the 1950s.












This unique design eliminates the need for antenna matching baluns and can
be fed directly with 50 Ohm coax.




The DOUBLE BAZOOKA is 98% efficient and typically provides S.W.R. readings
of less than 2:1 over the entire amateur band.




Since this antenna has no exposed metal wire static charges can not build
up thus reducing noise by 6dB over antennas constructed of exposed wire.




The DOUBLE BAZOOKA antenna will handle full legal limit power with no
effect to performance.




The DOUBLE BAZOOKA is recommended to be mounted in an inverted V
configuration for optimum results. However the DOUBLE BAZOOKA can be
configured horizontally with equally good results.




The 80 Meter DOUBLE BAZOOKA antenna will operate on 80 through 10 meters
with the aid of an antenna tuner.




The following chart gives overall antenna lengths and recommended height
placement above ground for a single DOUBLE BAZOOKA.



Band ANTENNA Center height End Height
Lenght above Ground above ground

17 M 25.4 feet 25 - 35 feet 7 - 10 feet
20 M 32.4 feet 25 - 35 feet 7 - 10 feet
40 M 64.0 feet 25 - 35 feet 10 - 15 feet
80 M 121.0 feet 40 - 60 feet 15 - 20 feet
160 M 248.0 feet 60 - 90 feet 15 - 20 feet




If erected as a Dipole this antenna has horizontal polarization.

This antenna can also be installed in an inverted V fashion Center
elevated, with 90 120 degrees between the legs Then it will have vertical
polarization and will usually out perform a dipole type antenna at
distances of over 500 miles due to its lower angle of radiation.

The coaxial dipole uses the same type of coax for the feed line as the legs
are made of or it will not work properly. Ill use the 75-80 Meter antenna
as an example.




To build one of these antennas resonant at 3.920 Mhz.




Measure out a 120 feet length of coax and cut,this will be the antenna. At
the center of this cut off 1 of cover all the way around and remove. Now
cut the shield all the way around and separate. Do not cut the dielectric.
This is the point at which you will support the antenna so make a hanger
from rope or other material.




Take your feed line coax and cut back cover at one end so as to be able to
separate the center conductor and shield. Attach the feedline center
conductor to one side and the shield to the other side of the antenna coax
shield at the center of the 120 foot long piece.




Next, measure down each leg 30 6 from the center and cut a slot in the
cover and shield, so as to be able to get to center conductor.




At this point solder the center conductor to the shield on both legs. Now
seal the center point and each leg where you soldered, with nonconductive
silicon, so as to make weather tight. At the end of each leg strip the
shield and center off and solder shield to the center conductor so it will
not separate and seal.




The center conductor from leg to leg acts as a balun, thus making this
antenna able to operate with a very low SWR across the whole band. 40 Meter
Addendum I have NOT personally built these antennas.




I was informed by KB5HOV that using my design he made two 80 mtr and two 40
mtr double bazookas using RG-8 for the entire antennas when he constructed
the 40 Mtr bazookas he had to add 18 inches to each end section to get them
resonant in the phone portion of the band.




This was true on both 40M antennas but after the extra 18 length was added
they were 1:1 VSWR at 7.215 Mhz and 1.1:1 VSWR at 7.295 Mhz.



Double Bazooka Dipole Antenna Version #2





This adaptation used in amateur radio only uses coax for the broadbanding
portion of the antenna, while the remaining portion of the elements are
constructed of twinlead or ladder line (see attached sketch below). Ladder
line is preferable for its inherent strength.

This is a single band antenna. It will not radiate harmonics of your
operating frequency. In addition, there is very little feedline radiation,
which is great for those who have problems with TVI. Its broadband
characteristic makes it ideal for 80 meters and 10 meters.

The Bazooka antenna consists of a half- wavelength of coaxial line with the
outer conductor opened at the center and the feedline connected to the open
ends. The outside of the coax and the ladder line operate as a half-wave
dipole. The inside of the coax elements, which do not radiate, are
quarter-wave shorted stubs which present a high resistive impedance to the
feed point at resonance. Off resonance, the stub reactances change in such
a way as to cancel the antenna reactance, thus increasing the bandwidth of
the antenna.

At the very center of the coax carefully cut away about one inch of the
outer vinyl jacket.

Then cut the exposed shield all the way around at the center of the exposed
area. Be careful that you do not cut the dielectric material or the center
conductor in the process. Twist the two pieces of exposed shield into small
pig-tails. These are the feed-point terminals for the antenna. The center
conductor of the feedline is soldered to one and the shield of the feedline
to the other. Now solder the center conductor and shield together at each
end of the coax antenna element. Solder the two ladder line wires to the
end of the antenna element. At the other end of the ladder line, solder the
two wires together.

The two conductors at each end of each piece of ladder line should connect
to each other.
Dont connect one end to the other end of the ladder line
Think of it as connecting the two conductors in parallel

Use a square piece of plastic at the antenna center, drilling a small hole
on each side of the coax, wrapping a small wire around the coax and through
the holes and twisting the wire together on the other side. A small amount
of quick setting epoxy secures the coax to the plastic support and prevents
the wire from untwisting.

A coating of silicone rubber or epoxy seals and protects the feed-point
from the weather. Do the same where the ladder line is soldered to the
shorted end of the coax.

Short the center to shield at each end of the coax. Short the twin lead at
each end and solder the coax/shield junction to the coax. Use 50 Ohm Coax
feedline at least 66 feet long.



_ _____| |_____ _
[]- [_______________________]- [_]
Twin-Lead * * * * * * * * * * * COAX * * * * * * * * * * * Twin Lead
---------L=325/Freq---------
coax length
-----------------------L=460/Freq-------------------------
Total Overall Length
Total Overall length minus Coax length = Total Twin lead length.
Example: 3.888 Mhz.
Total length 118.3 feet
Coax length 83.6 feet
Twin lead length 34' 8 1/2'' feet total or 17' 4 1/4'' on each end of the
antenna.





Addendum
Added December 2004
I made this page quite some time ago.
This is ALL the info I now have on Double Bazookas.
I have NO other formulas, info, etc.





Please DO NOT eMail me with questions about Double Bazooka antennas.




All Copyrights to W2HT article originally available at
http://www.bloomington.in.us/~wh2t/bazooka.html

The post Double Bazooka Coaxial Dipole Antenna appeared first on IW5EDI
Simone - Ham-Radio.


///////////////////////////////////////////
HF Ham Radio Antennas For Apartments

Posted: 14 Apr 2020 04:14 PM PDT
http://www.iw5edi.com/ham-radio/3835...for-apartments



by AG4DGÂ*



The Issues




A full-size antenna for HF (except for the 10m band) is not feasible for
apartments due to the space required. An apartment antenna should have the
following characteristics:







Compact Size: This is the most obvious requirement of apartment antennas.
As an apartment dwelling amateur radio operator, you want to keep a
relatively low profile in order to minimize complaints from neighbors and
the landlord/management. Except for the very highest HF bands like 10m, a
full-size antenna would be conspicuous and even intrude into common areas
and other peoples property.Multiband: Due to limitations in storage space
and an aversion to overloading my home with stuff, I prefer multiband
antennas. Of course, if your only transceiver is a singleband transceiver,
then a singleband antenna would make more sense.Easy to set up and take
down: If you are on the ground floor or neighbors can easily see your
balcony/patio, you want to remove your antenna when you are not operating.
If your antenna is always visible, this presents a security risk (on the
ground floor) AND increases the chances that neighbors and/or the
landlord/management will object.Cost: I want to operate HF that offers the
most bands for the least cost. In other words, I want the most bang for the
buck.



Popular Apartment Antenna Ideas That Didnt Work For Me



There may not be trees in the right locations.You need two wires, not one,
because a random wire antenna needs a ground plane to act as the second
half of the antenna. And no, the two wires cannot be taped together. (Have
you seen a dipole with the two halves taped together?)Too difficult to set
up and take down: Setting up a wire antenna requires shooting it into the
air with a slingshot or other device. This can take several minutes.
Likewise, it also takes too long to take the antenna down.Too conspicuous:
The process of setting up the antenna and taking it down could attract the
attention of people. Also, the antenna isnt confined to your own property;
it likely would intrude into common areas. If you live on the ground floor,
the antenna could also get in the way of people walking by.Coax Length: You
might need VERY long coax to try this idea. Since my apartment points away
from the parking lot, I would need a few HUNDRED feet of coax.Conspicuous:
This would also be a VERY conspicuous setup. The coax would likely get in
the way of people walking by (unless you are parked directly in front of
your apartment and there is no sidewalk between your car and the
apartment). Also, your antenna and the coax would be easy to spot. In some
cases, the coax would cross part of the parking lot, and people would have
to drive over it.In most apartments, no room is big enough to accomodate a
full-size wire antenna (except perhaps for the highest HF bands) without
making it double back on itself (which makes it less effective).Parts of
the antenna will be TOO FAR from the exterior of the apartment and thus
reduce the signal received from outside while increasing the noise received
from inside. This effect would be particularly pronounced in a stucco
building or other metal structure.Finally, THIS IS THE PERFECT RECIPE FOR
RFI. Think about this. If you use a small antenna that is confined to just
one part of one room, most of the RF field will be confined to that one
place, and the rest of the apartment will be relatively free from RFI
problems. But if you string the wire antenna all over the apartment, MANY
PLACES are close to the antenna, and there will be RFI galore. When I tried
this solution, I kept tripping the circuit breaker even at the lowest power
levels. At least the RFI from the indoor slinky dipole antenna that was
confined to just part of my bedroom was more controllable. In this case,
all I had to do was unplug the lamp in my bedroom or plug it into an RFI
filter.It didnt perform adequately unless it is stretched almost all the
way out.Fragility: The slinky coils do NOT lend themselves to being thrown
onto a tree.Supports: This antenna requires MANY supports. You need a
support (1) on which to hang the center of the antenna, a rope inside both
coils (1+2=3) to support them, and supports at both ends of the antenna
(4). Then you have to stretch out both slinky coils and secure the ends of
the coils (4+2=6) to hold them in the stretched position.




However, I cant completely dismiss these ideas, as one of them may work for
you.








Methods of Shortening Antennas




A full-size antenna (1/4 to 1/2 wavelength long/tall) is not feasible for
most apartment dwellers on HF. The antenna must be this size in order for
it to be near resonance. However, there are ways to achieve resonance with
smaller antennas by using loading methods that electrically lengthen an
antenna while keeping it physically short. This is NOT a free lunch.
Smaller antennas, although resonant, are less efficient and have a narrower
bandwidth than full-size antennas. So if you have the room for a larger
antenna, you should take advantage of this.



There MUST be good conductive contact between the capacitance hat and the
top of the antenna.Capacitance hats can use spokes, a single wire, and/or a
metal disc. I prefer a metal disc, as this provides more capacitance for a
given amount of space.The capacitance hat MUST be at the top of the
antenna. Mounting the capacitance hat lower makes the portion of the
antenna above inactive and has the same effect as chopping off the part of
the antenna above the capacitance hat. SO PLEASE KEEP THE HAT AT THE TOP!








Popular Apartment Antennas



Unless your antenna is at least 1/2 wavelength above the ground, it will
transmit most of the RF straight up instead of at low angles above the
ground. While this may work well for local contacts, this will handicap
your DX capability.If your dipole is made of wire, it must be 1/2
wavelength long. Unless you make a dipole out of Hamstick mobile antennas
(there is an adapter you can buy to do this), a shortened dipole may be
difficult to find or make.The loop antennas sold by vendors are VERY
expensive ($300 to $500) and only transmit on the higher bands. NONE of the
compact loop antennas is designed for 80m. Only one is designed for 40m,
and this is one of the priciest ones.Homebrewing a loop antenna is
difficult. Good variable capacitors are hard to come by, and having VERY
low conductor losses is VERY important due to the VERY low radiation
efficiency. Thus, building a loop antenna requires using a blowtorch or
other large and dangerous gadgets to bend a copper pipe and weld things
together. An apartment wouldnt be an appropiate place to try to build a
good loop antenna.Even on 80m, the Screwdriver antennas are almost as
efficient as the Texas Bugcatcher. (Note that both have large coils for
better Q.)The main selling point is the REMOTE tuning. A screwdriver motor
(hence the name of the antenna) that you control with a switch moves the
coil up and down in order to adjust the inductance loading. This gives you
flexibility in the height of your antenna and makes the antenna
CONTINUOUSLY tunable FOR RESONANCE.NO tuner is needed, because the Z-match
built into the bottom of the antenna matches your antenna to provide a 50
ohm impedance at the coax feedpoint.Best of all, this antenna allows you to
use every band from 10m through 80m and is priced at only $160. If you use
a tall enough antenna and a large enough capacitance hat, you can probably
even work 160m as well.

The post HF Ham Radio Antennas For Apartments appeared first on IW5EDI
Simone - Ham-Radio.


///////////////////////////////////////////
Full Wave Loop Antenna

Posted: 14 Apr 2020 02:56 PM PDT
http://www.iw5edi.com/ham-radio/3831...e-loop-antenna



by WH2T




I have personally used a 160 meter band Horizontal Full Wave Loop antenna
with very good success. I worked all 50 states and several countries with
100 Watts using the antenna on the 160 meter band. With a good antenna
tuner the antenna will work 6 through 160 meters. I currently use a 75
Meter Full Wave Delta Loop on the 6 through 75 Meter bands with a tuner. I
am very pleased with it!





These Omnidirectional antennas offer Horizontal polarization, and about 2.1
dbd of gain. They are much quieter than a dipole or a vertical, have a
broader bandwidth and will usually out perform a dipole antenna.













Feed 1 wavelength of wire with a piece of either 50 or 75 ohm coax. I am
using 75 ohm because I had some.





To determine the approximate length in feet of a Full Wave Loop antenna use
the formula




1005/Freq in Mhz = length in feet.




For 160 meters a full wave loop antenna resonant at 1.9 Mhz would be about
529 feet long.





For the 75 meter band a full wave loop antenna resonant at 3.85 would be
about 261 feet long. A full wave for 3.9 MHz = 257 Feet 8 inches.









These length antennas may require some pruning or trimming of the wire to
obtain a low VSWR. But if you measure carefully you should get very close.




Do not connect the antenna wire to the tower on this antenna.

This type of antenna doesnt depend on an efficient ground system for
efficient performance. The impedance of a Full Wave Loop antenna is
theoretically in the vicinity of 100 ohms.

Connect one end of the wire to the coax center and Connect the coax shield
to the other end of the wire. Be sure and seal the end of the coax against
water. Form the wire in a loop and run it horizontally to trees or whatever
supports are handy. Be sure to insulate the wire from the supports.

Extend from the wire with ropes if necessary to reach your supports.




On the higher frequency bands a full wave loop antenna can be oriented as a
vertical diamond or vertical cube. Fed at the top or bottom corner of a
diamond this antenna has horizontal polarization.

Let me clearly state this Fact.

A vertically oriented horizontally polarized one wavelength loop antenna
with the bottom of the loop 1/2 wave length above ground is a FINE antenna.

Do NOT put the bottom of the vertically oriented loop more than 1/2 wave
length above ground.



The post Full Wave Loop Antenna appeared first on IW5EDI Simone - Ham-Radio.