[TenTec] Bazooka antenna.. More than you wanted to know!

[Dr. Gerald N. Johnson]

On Mon, 2008-06-02 at 23:31 +0100, Steve Hunt wrote:
> Jerry,
> 
> Very interesting stuff! I had not looked at your "parallel-tuned-circuit 
> in parallel" option before; I just modelled it in EZNEC to learn more 
> and saw some interesting results.
> 
> I took a dipole constructed of 128ft of #14 copper wire. It resonated in 
> Free Space at 3.745MHz, where it exhibited a resistance of 74 Ohms. It 
> had a 2:1 VSWR bandwidth of 150KHz.

I chose 120 feet, I think. My tuned circuit resonated at a higher
frequency and the SWR was never 1:1. It was a circle, that when shifted
through 5/16th wave (or 50' in real coax with Vp of 0.66) of 72 ohm coax
gave a SWR about 1.3:1 from 3.6 to 4.1 MHz, a 500 KHz bandwidth. The
band ends are quite clear, the SWR rise is rapid on both ends of the
working range. That's not computing for any coax losses. I developed
that design before EZNEC using only a book of dipole impedances, a
calculator (maybe a slide rule), and a Smith chart. The tuned circuit
was 3 meters of RG-58 and a 2000 pf capacitor, though I think I computed
a 2200 pf was better. Tuned to resonate at 3850 KHz with the dipole
resonant below that.

Users have found the match better when there is another 50' of 50 ohm
coax between the shack and that 5/16" wave 75 ohm transformer.
> 
> Then I looked to see how the Susceptance changed for a small frequency 
> shift, and calculated the component values you would need in the 
> parallel tuned circuit to compensate. I was surprised how big a C was 
> needed (5,730pF) and how small the L (0.315uH). I tried that combination 
> in EZNEC and the VSWR bandwidth improved to 185KHz. What was interesting 
> was that, although the reactive element was being compensated quite 
> accurately, the presence of the tuned circuit caused the 
> series-equivalent Resistance to vary more quickly with frequency than in 
> the un-compensated case. So the bandwidth improvement was not as great 
> as I'd expected.
> 
> Next I tried your 2000pF figure and the 3m (lossless) stub. It didn't do 
> quite as well as my values, so I doubled the capacitance and shortened 
> the stub to compensate. The bandwidth was then back up to 185KHz.
> 
> Here's the interesting bit ..... only when I started to introduce some 
> losses into the stub did the bandwidth increase really significantly. 
> Once I put "real world" RG58 loss numbers in, the bandwidth shot up to 
> 400KHz. But at that point the radiated power was down by 2dB!

Those who prize the leaky dummy load of the B&W broad band dipole never
miss 20 dB loss in efficiency at some frequencies while gaining instant
frequency agility.
> 
> Using RG213 loss figures produced a bandwidth of 305KHz and a drop in 
> signal of 0.9dB

Then being around houses, other antennas, and real earth introduces a
lot of loss while the signal strength over any given real RF patch
varies sometimes at the rate of 10 dB / second which sometimes makes a
loss of 0.9 or 2 dB not consistently measurable.
> 
> I guess only individual ops can say whether these losses are prices 
> worth paying for the convenience of "tuner-free" operation.
> 
> 73,
> Steve G3TXQ
> 
I'm using a plain inverted V with a tuner from 80 (and have made a
contact on 160) through 6m. 12 gauge copper wire fed with RG-141. It has
gone intermittent so I need to rebuild it, probably replace the feed
line that I suspect has broken the solid center conductor while flexing
in the breezes that sometimes hit 80 mph around here. Maybe I'll put the
TDR on it to see where and how often its broken.

Another way to broaden an 80 meter antenna I applied at the Ft. Belvoir
MARS station, K4WCC back about 1968. Their 80 meter dipole was cut for
3900 or 4000 for use up to 4025 KHz. I wanted to use a spare S-line to
work my dad (K0CPN now SK) on about 3.6 KHz. Sometimes I used a Viking
500 and NC-303 which tuned the off resonant antenna far better than the
Collins. The dipole was an inverted V. I took down the ends, attached a
second wire about 6' from the original insulator. Then I spread those
wires with a 6' or so bamboo pole (left over from a cubical quad used
there sometime in the past) and tied the original support rope to the
center of the bamboo pole. That pole bowed considerably but was an
insulator. That moved the resonant frequency of the antenna down to
about band center and the SWR at 3.6 and 4025 was under 2:1 which the
30S-1 drove quite well. Though my added wires were only 18 or 20 gauge
insulated hook up wire, I don't think it was losses that broadened that
antenna, it was the fat ends that were a first approximation to a
conical dipole.

73, Jerry, K0CQ