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HF NVIS 101

  • 15 Oct 2017 5:28 PM
    Reply # 5315884 on 4968034
    Chris Doering (Administrator)

    I'll be running the ATAS 120 for the Nov HF op session, but Rick mentioned allegator clipping a 30' section of wire to the antenna for a makeshift NVIS. I'm open for more ideas as the ATAS 120 is the only HF antenna I have.

    As a last resort could I simply adjust the mount on the ATAS 120 and aim it near vertical or is that not a good idea?

  • 15 Oct 2017 8:19 PM
    Reply # 5316168 on 4968034

    I have an ATAS-120 as well, so we can try both (I have an antenna switch)

    I am not  sure of  running the AtAS sideways, you usually need a peer connected to ground to be effective.

    That's my experience anyway.


    Will you be posting an on-air schedule and frequency?


  • 16 Oct 2017 7:13 AM
    Reply # 5316628 on 5316168
    Deleted user
    Donald Bower wrote:

    Will you be posting an on-air schedule and frequency?

    Yes, we will post a schedule a couple of days before the event. More than likely it will be on 40 meters.


  • 16 Oct 2017 8:52 AM
    Reply # 5316934 on 4968034

    I'm very curious to see how the Atas 120 will perform in NVIS, since it's only down to 40 meters. I think 60 or 80 meters would be better, but who knows... that's why we experiment.

    I've seen videos on youtube about using successfully a longer (semi)horizontal wire as an NVIS element. 

  • 17 Oct 2017 7:08 AM
    Reply # 5318384 on 4968034
    Deleted user

    I use a 135 foot inverted V antenna at our base camp. Works for both NVIS and skywave. It is just wire fed in the middle.

    The maximum NVIS usable frequency is always changing. You want to be as close as possible but not over. I use this map and some other graphs.

    http://www.sws.bom.gov.au/Images/HF%20Systems/Global%20HF/Ionospheric%20Map/WorldIMap.gif

  • 20 Oct 2017 8:39 PM
    Reply # 5324403 on 4968034
    Anonymous

    Having run the ATAS on my F150 for several years, it's worked marginally for me when I've been up in the mountains...and generally only works (other than line-of-sight) when I'm away from large cities. Of course, the shorter the wavelength, the more efficient, and the better the signal reports, with the ATAS antenna.

    I've also used a simple 20m dipole, up only about 20 ft., connected to the F150, with some okay results (probably more due to the low height than anything else).

    Over the years, I've poked about the internet, and chatted with other hams about the options. I almost bought the Hi-Q, but held back after a review from someone who knows the product well.  Other considered have been the Tarheel, and I've tried the Hamsticks with limited success.

    One recurring mobile antenna design I've seen during research has been the loop.  Some increase in efficiency may offset the "ugly." 

  • 21 Oct 2017 1:21 PM
    Reply # 5325015 on 4968034
    Chris Doering (Administrator)

    So9 I met someone who has an ATAS 120 and he removed the whip (carefully) and added to the length. I"m going to ask him if he can do NVIS with the longer whip.

    Any thoughts or comments would be interesting. Also I was wondering if the HI-Q or Tarheel antennas mountained in the middle of the vehicle directly over head would be the best. 

  • 22 Oct 2017 9:59 AM
    Reply # 5325668 on 4968034
    Anonymous

    To start off, I apologize if my inner engineer starts getting carried away in my posts. Sometimes that part of me gets carried away :)


    Adding length will no doubt help in overall radiation efficiency. Electrically, I'd go with the longest I could get away with (without exceeding 13-1/2' overall vehicle height, of course). I've read somewhere that certain whip lengths may tune better than others, but I haven't seen any empirical data to verify this.  Mechanically, one must also consider the moment/torque that the longer whip (e.g. wind load) would put on the whip connection point.

    As for mounting, for NVIS, putting the screwdriver* antenna on the roof would also help, by getting it up higher and maybe adding a little ground plane.  Since the RF radiation from the whip is perpendicular to the whip itself, we may wish to experiment with angling the whip closer to horizontal, thus radiating the signal to the clouds, where it can be reflected back to earth within our operational area.

    With the ATAS, mounting the SO-239 connector on a vertical roof rack member, and then cradling the coil section (as close to the whip as possible for mechanical stability) would be an idea.  I may try this on my pickup this week, in preparation for Nov. 11.

    They (usually the manufacturers) say that larger the base coils of a Hi-Q, or Tarheel or similar mechanically-tuned mobile antennas, mean higher efficiency.  I've heard that the "sweet spot" is a 3" dia. coil.  Again, I haven't (so far) read any non-manufacturer's white papers with empirical data..not that it's not out there.  I think that regardless of the manufacturer, go with what you have, until it breaks or you're tired of it, then "upgrade."

    -Clint

    *I use this term loosely to describe mechanically-tuned vertical antennas...it doesn't intend to offend those who own antennas that aren't actually "screwdriver" antennas (e.g. Hi-Q).

  • 23 Oct 2017 7:31 AM
    Reply # 5332571 on 4968034
    Deleted user

    Adding length to a screwdriver just means less coil needed. For example when I put on a 19 foot whip on my screwdriver rather than the 6 foot whip I can tune 80 by barely adjusting it. Of course I lose 10 meters and probably 12 too.

    Ideally an antenna just wants to be a least a 1/4 wave long and the coil just makes up the difference (with losses).

    A longer antenna will always be better but probably not help NVIS much.

    As far as NVIS freqquency, the rule of 40 for day and 80 for night is not bad but might not always be true. As I write this the maximum usable freq for NVIS at 7:23am is 4Mhz. I am pretty sure it will follow yesterday, so by about 9am the maximum usable freq for NVIS will be around 6-7Mhz. So why not just use 80 meters all day and night? Becuase we need to be as close to the maximum usable with out going over.

    I think Clint mentioned also the screwdrivers are pretty good antenna but will never do NVIS without some type of horizontal component. That is why a vertical dipole works or an inverted V or a piece of wire clipped to the screwdriver or a bent over whip (I am not convinced on the bent over whip). I will bring all of these to the HF Recon so everyone can see in person what works and what doesn't.

  • 24 Oct 2017 10:07 AM
    Reply # 5335300 on 4968034
    Anonymous

    Looking around the internet, reading both military and commercial documentation, I'm definitely warming up to Rick's setup as being the standard.  USMC docs state the usable NVIS frequencies are between 2 MHz-12 MHz (80m, 60m, 40m for ham bands), with a mobile antenna setup using a dipole, longwire, etc., being quickly deployable & taken-down.  For our purposes, I think that what we call our base would be considered mobile from an HF point of view.

    As for a vehicle antenna system, I'm seeing that the balance between durability, cost, and complexity is difficult to meet. A 16 ft. whip, bent over almost horizontally, is very durable (compared with "screwdriver" styles).  However, this requires some sort of tuner to match -- thus increasing system complexity.  Screwdrivers can help avoid using a tuner, but requires a more mechanically complex (and thus prone to failure) design.  Regardless, vehicle-mounted antennas can assist for NVIS, but is typically used when line-of-sight (VHF or UHF) cannot be used.  If I could get my hands on a 16 ft. whip and mount, heck yeah I'd mount it on my truck.

    One more thing that Rick's has proffered is HF digital. Other than satellite comms, it appears that (at least in the old manuals I'm reading) HF digital is the mode to use where possible for NVIS. While vehicle-to-base can continue to be verbal/phone, standardizing on a digital mode may be best to ensure the most accurate transfer of information during multi-jurisdictional operations.

    Just my thoughts.  -Clint (NR6T)

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