I got a Double Bazooka!
https://ultimax-antennas.com/store/p4/DOUBLE_BAZOOKA_SINGLE_BAND.html
I like it. Here are a few details that may be of interest:
I put the antenna up at about 45 feet in a slightly inverted V, which is how the manufacturer recommends for this antenna. This will generally give a more unidirectional pattern and lower the feed-point impedance a bit closer to 50 ohms. ( A flat dipole is about 72 ohms at the feed-point )
I am feeding this antenna with a little over 100 feet of DX Engineering DXE400 coax. ( Virtually identical to Times Microwave LMR-400 Ultra-Flex ) All of the measurements include the feed-line, the lightening arrestor, and all cabling through the amp, tuner, and radio selector switch. The analyzer sees exactly what the radio sees.
Here is how it looked out of the box:
I noticed right away that the reputation of the Bazooka antenna of being broad-banded was well deserved:
With this information we see that the antenna has a 2:1 bandwidth of 194 MHz, and a 3:1 bandwidth of 346 MHz. That is not too bad.
Where to tune and how to do it?
We are off to a good start, but our new antenna won’t do us much good tuned for 3.476 MHz.
In order to make the antenna work better where we want to use it, we need to shorten it up a bit. I will use this antenna mostly in the 75m portion of the band so I will try to tune for about 3.850 MHz.
If you were a CW operator you might go for the bottom of the band like 3.550 MHz or so, FT8 on 3.573, etc.
So how do we know how much to shorten our antenna and what do we do with the “extra” wire?
Let’s take the first question first: how do we know how much to shorten the antenna? The answer is pretty simple really, we know already that our antenna is 134.64′ long electrically. (I didn’t measure it physically) We know this because our first sweep with the antenna analyzer says the wire is resonant at 3.476 MHz.
To see how much to take off: (468/(our desired resonance)) will give us the target length.
468 / 3.850 = 121.56 feet. Our antenna is currently 134.64 feet long so: 134.64 – 121.56 = 13.08 feet.
Now divide this number by 2 and you know how much we must shorten each side of the antenna:
13.08′ / 2 = 6.54 feet from each side. Be sure to take the equal amounts from each side so our antenna stays symmetrical.
So what do we do with the extra wire?
If this antenna were made from regular wire, you might be temped to cut off the excess to keep the ends nice and tidy. But since our Double Bazooka is made of coaxial cable, we would have to re-terminate the coax and we would then be stuck with our length forever.
Instead, let’s just fold it over and lash it to itself. This will affect the tuning, but only a little bit. Use a few zip ties and some black electrical tape until we are 100% tuned and done.
How does it look after shortening the antenna?
Before we look at the new plot, I should say that I changed a few things since our last measurement. Ideally you would not make changes mid-stream but sometimes you do.
Here is what changed: I raised the feed-point of the antenna another 7-10 feet and increased the angle of the inverted V. Not by a lot, but enough to throw off our calculations as you will see:
So you can see two things have thrown off our calculation: the fold over of the excess cable, and the increase in the inverted V which altered the feed-point impedance.
We are not too far off though, and one more adjustment should get us where we want to be.
How is our bandwidth?
Let’s talk about the term bandwidth and what it means in the context of an antenna.
Here are our numbers after the first adjustment:
Resonant at 3800 kHz, 2:1 bandwidth is 213 kHz, and the 3:1 bandwidth is 403 kHz.
For context, here are the numbers for my old center fed doublet made from 14 AWG coated wire on 80m for comparison: 3:1 bandwidth was 250 kHz, 2:1 bandwidth was 150 kHz.
Match that to our Double Bazooka numbers and you can see a mono band antenna made from RG-8x coax can be a very good performer indeed. Fans of the Double Bazooka design claim they are wide banded, which is supported by our measurements above, and that they are quieter than a regular dipole due to their closed circuit design similar to a loop.
I am unsure if the Bazooka is quieter than my old Doublet since I only have room for one of them at a time. However, I am prepared to say the Double Bazooka has a better signal to noise ratio.
For all intents and purposes, this antenna is usable in the entire phone portion of the the 80 and 75 meter bands. That is pretty impressive. It also hears very well, and can take 1500 watts continuously.
Signal reports are good!
The final trim and results
Instead of adjusting the overall length to shorten our antenna (electrically), I am going to first try lowering the feed-point a little bit. ( Flattening the inverted V ) This will have two consequences, one it will alter the feed-point impedance which will change our point of resonance, and two, it will increase gain some. I am also hoping there will be a sweet spot for the lowest noise on receive, aka a higher signal to noise ratio.
At the height at which most of us are running our 80m antennas, ( 55 feet ) a few feet one way or another isn’t going to make a difference in your radiating pattern, but there might be a height close by that is a little more isolated from power line noise etc.
( insert final ant plot and resonant freq data ) ( Will update soon )
Conclusions
If you are still confused about all this bandwidth talk, let me boil it down a bit more. The ARRL Antenna Book states that there is basically no difference as far as losses are concerned from a 2:1 SWR on down to a 1:1. In other words, any operation within the 2:1 bandwidth can be considered the same as resonant as far as operating is concerned. You will still want to use a tuner in most cases to protect your radio or amp finals but your losses will be minimal.
Most radios ( and linear amplifiers ) can tune into a 3:1 SWR give or take a little bit. This means that for all intents and purposes a 3:1 bandwidth can be considered usable with just about any radio. You should choose wisely then where to resonate your antenna based on where you are most likely to operate. In our case, we tuned the antenna for the 3.8 MHz part of the band, so, if we want to work FT8, we may have a problem.
At the 80m FT8 frequency, we are seeing a 3.67:1 SWR on our double bazooka. This is too high for most transceivers built in tuners and also too far for many tube amp output networks to manage. So what if we use a wide range tuner to tune the antenna at 3.573 MHz?
The problem here is that the Double Bazooka is a coax fed antenna. As such, we need to be concerned about higher SWR in the feed-line. Coax tends to have more losses at higher SWR and it gets worse if you are running power at your station.
If you are running 100 watts or less, you will have about 10-15% power loss in your LMR400 at a 3.67:1 SWR. That is about 0.434 dB per 100 feet in addition to the .8 dB loss in the cable itself. That is a substantial increase in loss vs a 1:1 SWR and it will get worse with higher power and SWR levels.
Since I never run more than 50 watts in FT8, this will not be an issue for me. If you run more power than that, take these losses into account.
I would not personally run full power at this high of an SWR. In the CW portion of the band, we are hitting more than a 4:1 SWR and that is pretty high for coaxial feed-line.
I do think you can use this antenna and feed-line system on the entire band but I would de-rate the power levels accordingly. If you are running 100 watts maximum, you will not have any issues in all likelihood, but beware of QRO outside the 3:1 bandwidth.
If you want an antenna to regularly work the CW, digital, and phone portions of 80 meters, you might feel limited by the double bazooka. A Doublet is always a fine choice when maximum frequency adaptability is in order, but the Double Bazooka is a very fine antenna and one which I will be using far into the future.
I highly recommend you try one!
Addendum: this article will most likely never be finished because a storm came by and snapped off a big branch which utterly destroyed the antenna. I am undecided if I will try another one since I have a good doublet already and I am also about to put up an 80m full wave loop. I did buy another one and will complete the review soon.
So the final tune will never happen. It was a good antenna.