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4/18/2021 9:59:29 PM
Posted: 4/19/2017 6:49:05 PM EDT
If you are putting together and trying to tune a 40m dipole using hamsticks for NVIS and or a 20m telescopic vertical with 4 wire radials and trying to tune these antennas with an antenna analyser don't put them up in your driveway between 2 cars. 10 feet is not far enough away for low mounted antennas. And if you do happen to do this recognize right away why this is a bad idea when it doesn't work, instead of wasting 4 hours of your life wondering what could possibly be wrong and cursing MFJ along the way..

On the other hand, if you want to tune a 20m vertical to 12 mhz with a low SWR taking advantage of coupling to your 2 vehicles, by all means continue.

That is all.

M    A    C     H     spells dumbshit.
Link Posted: 4/19/2017 8:15:43 PM EDT
Oh man. Been there before.
Least you figured it out!
Link Posted: 4/19/2017 8:21:21 PM EDT
The time was not wasted. You learned something in the process.

BTW, the NVIS antenna made of 2 Hamsticks will be very inefficient. It's hard to guess the efficiency without modeling but IMHO, you'll be lucky to get more than 10 Watts ERP ( effective radiated power) with 100 Watts out or the radio, on 40 meters. That's right, 90% (or even more) of RF power will be wasted in the shortened antenna and ground losses.
If possible, it's best to use a full size dipole. For NVIS,  place at least one reflector made of a single wire (1/2 wave +5%) on the ground to improve efficiency.
Link Posted: 4/19/2017 11:35:20 PM EDT
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Quoted:
The time was not wasted. You learned something in the process.

BTW, the NVIS antenna made of 2 Hamsticks will be very inefficient. It's hard to guess the efficiency without modeling but IMHO, you'll be lucky to get more than 10 Watts ERP ( effective radiated power) with 100 Watts out or the radio, on 40 meters. That's right, 90% (or even more) of RF power will be wasted in the shortened antenna and ground losses.
If possible, it's best to use a full size dipole. For NVIS,  place at least one reflector made of a single wire (1/2 wave +5%) on the ground to improve efficiency.
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I did do that,

Got the best z and SWR with the ham stick dipole at 6 feet with  2 grounded 1/4 wave radials. made a contact in MD from MA but at S4 and that was between the cars! I want an NVIS at 40m that is compact and easy to put up without taking up a lot of space, ford such a think exist?
Link Posted: 4/20/2017 1:06:39 AM EDT
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ford such a think exist?
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Huh?
Link Posted: 4/20/2017 7:17:47 AM EDT
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Quoted:


Huh?
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Autocorrect.  FORD = DOES and THINK = THING.
Link Posted: 4/20/2017 7:32:01 AM EDT
FWIW, here's a datapoint to consider:  the professionally demo'ed NVIS systems I've seen (usually run in the 4 or 5 MHz range) were inverted-v's around 25 ft tall with autotuner / balun at the feedpoint.  All running up to 200 W wideband (25 kHz?) digital.  I imagine you could afford to knock a few dB (perhaps 6 dBb?) off of that in allowable loss for SSB or narrowband digital.

They were a little more robust and taller versions of this http://www.kpowmedia.com/nvis/


As others said above, being too near ground increases ground losses, so it's to your advantage to hoist it up a few tens of feet.  An extra step you might experiment with would be to extend the length of the radiator wires (allowing less inductance to be used and higher radiation resistance), so that the coil losses would be small compared to normal ground losses.
Link Posted: 4/20/2017 9:16:34 AM EDT
Link Posted: 4/20/2017 10:22:04 AM EDT
You will never forget that lesson.  Definitely learn more when you start to setup than learning questions.  At least you didn't hurt anything..    good story...

Prosise
Link Posted: 4/20/2017 10:48:54 AM EDT
MFJ-1979? let me know how you like it. i have the 10ft version but thinking of grabbing the 1979 for a new portable antenna design.
Link Posted: 4/20/2017 10:54:10 AM EDT
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Quoted:
You will never forget that lesson.  Definitely learn more when you start to setup than learning questions.  At least you didn't hurt anything..    good story...

Prosise
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    ^ This. All of this.    

      OP, well written, made me laugh.  Live and learn, huh?
Link Posted: 4/20/2017 11:13:18 PM EDT
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Quoted:
MFJ-1979? let me know how you like it. i have the 10ft version but thinking of grabbing the 1979 for a new portable antenna design.
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I put it on a small camera tripod yesterday in my back yard, put out 4x22 ft radials, tuned it to 20 meter voice band with 1.4 SWR and made DX contacts in Spain and Hungary from MA with 100 wats on battery power. Looks like a great camping antenna.

For a 2 foot collapsed $60 antenna and a small tripod with no coil loses I think it is a great deal. I also tuned it to 15 and 10 meters but the bands were dead.

It is a keeper. I found a company that makes an adjustable coil that tunes it to 40 and 80 but it obviously will not be as efficient

edit: it resonates at 75 ohms with horizontal radials like every other vertical, I know there is a way to match that with 50 ohm cable but I don't remember and need to look it up, That would clean up the swr.
Link Posted: 4/20/2017 11:49:44 PM EDT
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good article thanks.

.I had no idea that HF could be deirection found
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