Posted: 4/19/2017 6:49:05 PM EDT
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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. |
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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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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. 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? |
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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. |
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This is what you seek.
https://brushbeater.wordpress.com/2016/08/15/nvis-explained-i/ |
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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. 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. |
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good article thanks.
.I had no idea that HF could be deirection found |