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Author Topic: 160 Meter Inverted-L Bazooka Antenna  (Read 5040 times)
KM5PS
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« on: November 29, 2011, 06:30:40 AM »

160 Meter Inverted-L Bazooka Antenna by  website http://iacantennas.com/160-Meter-Inverted-L-Bazooka-Antenna.10..
Does anyone have one of these antenna's?  Just wondering how they work.  I am thinking about making an Iverted L antenna for 160 meters, but came across this antenna and was wondering how they work?

73's
km5ps
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K1ZJH
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« Reply #1 on: November 29, 2011, 06:41:48 AM »

http://vk1od.net/antenna/DoubleBazooka/index.htm

Probably a variation of the above?

At best they seem to trade some gain for a very modest
increase in bandwidth. I like simple; the more you complicate a simple
antenna the more things to go wrong.

Pete
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K4SAV
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« Reply #2 on: November 29, 2011, 11:42:48 AM »

Quote from the website:
"A DOUBLE BAZOOKA antenna is an extremely broad-banded Half Wave Antenna that can operate efficiently across an entire Ham band with little change to the SWR."

Well that's not true.  Read the VK1OD link that K1ZJB gave if you want to find out how a double bazooka works.
Notice the very small bandwidth increase compared to a half wave dipole.

BUT, that's not what they are selling.  They are selling a QUARTER wavelength double bazooka.  You can make a half wavelength inverted L 160 meter antenna but that requires about 280 ft of wire (and a 150 ft support), or maybe a little less if you make it from coax.  The antenna they are selling is 138 ft long.  It also requires a good radial system (which they forgot to mention).

To get less than 2 to 1 SWR across all of 160 meters you will need a lot of ground loss. Of course that decreases the gain by a lot (not a good plan).  That may be why they didn't mention the radial system.

Also adding insulation to the wire DOES NOT reduce the noise level by 6 dB.

Also mounting the antenna 24 inches away from a tower is a very poor location, and could result in huge increases in SWR and power loss, depending on the resonant frequency of the tower.

Jerry, K4SAV
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W8JI
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« Reply #3 on: November 29, 2011, 04:33:33 PM »

People sure can be creative when writing advertisements. :-)

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W0BTU
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« Reply #4 on: November 29, 2011, 09:50:38 PM »


Many years ago, I built a 75/80 meter Double Bazooka like "Fig 6: Cross-connected Double Bazooka schematic (from ARRL)" on the above link. It was made from (and fed with) Belden RG-58C/U and 300 ohm line at the ends, with a coax choke at the feedpoint

Believe me, it was one of the worst antennas that I have ever tried.

Yes, the SWR was low enough, but my signal on other peoples' S-meters was even lower. :-)

It was 70' high and I fed it with a pair of 3-500Zs run hard. I made many, many OTA tests with it; other hams near my QTH with dipoles at only 30 feet running less power were consistently louder than I was to anyone anywhere, and we were at a loss to explain why that was the case.

When I finally replaced that antenna with a similar-length ordinary dipole --in the same space on my tower as that double bazooka occupied-- fed with open-wire line and a balanced tuner, I finally had a decent signal.
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KC9TNH
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« Reply #5 on: November 30, 2011, 04:39:35 AM »

People sure can be creative when writing advertisements. :-)
LOL.  Smiley Indeed.

 (that includes the erroneous stuff in the ARRL handbook under this topic, often quoted on the internet by folks who admit they haven't even built one)

The majority of articles I've seen on the net - including the poor design premises upon which subsequent disappointment/poor reviews are based - don't even account for velocity factor in the dimensioning of the thing. If one doesn't care about or do the detail work they are setting themselves up for remorse and should probably revise their expectations & build a more "standard" antenna. It's not all about gain; properly done they can be quieter while also hearing very well. If one's expectation is to toss away their tuner forever while listening to angels sing the premise of a bazooka probably isn't for them.


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73
Wes -KC9TNH
"Don't get treed by a chihuahua." - Pete
KM5PS
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« Reply #6 on: November 30, 2011, 04:48:44 AM »

Well I think that I will for now stay with my Inverted Vee antenna.  I will keep on looking for some more and may try a Inverted L later. 
Thanks to everyone who has responded so far. 
Gud Luck to everyone in the 160 meter Contest this weekend.  I will be on the air at a friends house.  We are planning on working the contest together this year. 

73's
John
KM5PS
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WB6BYU
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« Reply #7 on: November 30, 2011, 07:18:38 AM »

Quote from: KC9TNH
It's not all about gain; properly done they can be quieter while also hearing very well. If one's expectation is to toss away their tuner forever while listening to angels sing the premise of a bazooka probably isn't for them.


Actually the original purpose was wider operating bandwidth, not gain or low noise on receive.  All the
designs I've seen (and the ones I've used) included a correction for the velocity factor of the coax:  it
applies to the length of the coax stub, not the overall length of the antenna.  That's why, in this
variant, the coax stub extends about 2/3 the length of the antenna and the rest is made from twinlead
to form a fatter element.

Commercial variants such as the "Snyder Dipole" have been patented on the premise of wider bandwidth
by using 25 ohm coax.  Basically the two quarter wave coax stubs provide some reactance compensation
with a change in frequency - just like connecting a parallel tuned circuit across the feedpoint.  The
type with the crossed connections puts the two stubs in parallel, which gives slightly better bandwidth
than in series (the version where the coax center conductor is uncut at the feedpoint.)

But it is still a dipole, and will have the same gain and pattern as any other dipole (minus the losses in
the coax stubs.)

Low noise?  There is no way it can recognize which bits of RF are noise and which are signal, so it will
pick up the same amount of noise from the ether.  As W8JI documents on his web site, precipitation
static happens to insulated antennas as well as bare ones whether they have a DC ground or not.
The only noise reduction I would expect to see is that due to a difference in pickup via the feedline
when the antenna is changed (which would be eliminated by a proper balun anyway) or the overall
drop in all signals due to stub losses.

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K1ZJH
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« Reply #8 on: November 30, 2011, 07:18:47 AM »

The inverted L will give you a lower take off angle, and it doesn't have to be a Bazooka. 130 feet of wire
works well.

Pete
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K4SAV
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« Reply #9 on: November 30, 2011, 10:05:13 AM »

The inverted L will give you a lower take off angle, and it doesn't have to be a Bazooka. 130 feet of wire
works well.

Pete

Yes an inverted L is a very good antenna for 160.  It's about the best you can do when you have height limitations.  If you don't have height limitations then put up a 130 ft vertical.  A good radial system is the key to making this perform well.  Try to make the top wire as high as possible.  I would much rather have a vertical antenna for 160 than a horizontal one.  I can't say that about the other bands because height is not as big a problem on those bands, and then there is the earth's gyro frequency effect to consider on 160. 

Just don't make it a bazooka inverted L.

Jerry, K4SAV
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K1ZJH
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« Reply #10 on: November 30, 2011, 10:10:30 AM »

Jerry

If my antenna analyzer shows resonance at the expected frequency for my inverted L, may I assume
that any interaction with the support tower is negligable?

I have the inverted L up and supported by a cross arm on my tower. There was some question of
unwanted tower resonances affecting the inverted L.

Peter
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K4SAV
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« Reply #11 on: November 30, 2011, 03:56:27 PM »

Jerry

If my antenna analyzer shows resonance at the expected frequency for my inverted L, may I assume
that any interaction with the support tower is negligable?

I have the inverted L up and supported by a cross arm on my tower. There was some question of
unwanted tower resonances affecting the inverted L.

Peter

Not necessarily.  If the tower is very close to resonance on 160, then it is likely that you will never get the antenna to resonate with a reasonable impedance anywhere close to the right frequency.  However when the resonant frequency of the tower starts to come close to 160, all kinds of effects are possible. The base of the tower should be connected to the radial system for the L.  If you do that and the tower interacts, the effect is mostly a change in the feedpoint impedance and very little loss of gain (neglecting any feedline loss).  Well, actually that makes the assumption that the tower is just a single vertical element.  A guyed tower without broken guy wires might redirect the currents into the lower guy wires and that will result in a significant loss of gain due to high currents in wires low to the ground. The effect is mostly the addition of reactance into the feedpoint impedance.  That might make it impossible to obtain resonance or it might just shift the resonant frequency a little (or a lot) and change the real part of the impedance (that can go either way).  The same thing can happen when inducing any reactance at the base of the antenna.   You can even come up with a situation where the L is acting like a matching network for the tower (the real antenna) with the L doing very little radiating.  I mention that because I accidentally did that once.

If the tower is not connected into the radial system, then a lot of the current induced into the tower will be dissipated in the dirt.  However the SWR might be great.  Some combinations can produce a very wide bandwidth (due to double resonance) and a good match for coax.  The penalty of course is loss of gain.

It's difficult to be specific because there are all kinds of effects that can happen depending on the exact configuration of the antenna, tower, and other wires.  As a general rule, be sure the tower base is tied into the radials and all coax lines exit the tower at the base and the shields are tied there, and guy wires are broken up with insulators, and then if you obtain a usable SWR at the right frequency you should be OK.  Of course if you don't do these things you might still be OK depending on where things are resonant.

Jerry, K4SAV
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N4JTE
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« Reply #12 on: November 30, 2011, 03:57:20 PM »

Peter, I know you addressed your question to Jerry but if you are seeing" resonance" at the feedpoint then it sounds like you are good to go, as all relevant conditions seem to be addressed, what happens after your feedline length and position relative to ground radials etc will be the next challenge. As stated by myself in article about antennas myths, the bazooka is not worth the effort on any antenna system especialy an inverted L. Although they do advertise all over eham, hi
Regards,
Bob
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