Topics

NVIS

felix1063@...
 

Hello All,


Are there any ideas for a portable NVIS antenna for 75 meters that can be put up by one person on a flat parking lot without any high structures or light poles? 


Thank you,

--jeff

wb7aht

Mel Farrer
 

Well, the only answer is a pedestrian mobile system or a Alex type loop.


Mel, K6KBE

Andy
 

ai.egrpsAre there any ideas for a portable NVIS antenna for 75 meters that can be put up by one person on a flat parking lot without any high structures or light poles? 


Maybe something like this, built with PVC pipe:


Andy


Gary Nissan
 

Here's a Quick, Easy and Cheap NVIS antenna for roadside operations.    


[Gary Nissan]
ALWAYS SIGN YOUR POST WITH YOUR NAME / AND OR CALL SIGN
Moderators

Sent from my iPhone

On Mar 2, 2017, at 02:09, felix1063@... wrote:

Hello All,


Are there any ideas for a portable NVIS antenna for 75 meters that can be put up by one person on a flat parking lot without any high structures or light poles? 


Thank you,

--jeff

wb7aht

Carson VA3OSO
 

I use a linked dipole and a telescopic pole.  If you can find a way to support the pole you can setup anywhere.  

Carson
VA3OSO 


On Mar 2, 2017 2:09 AM, <felix1063@...> wrote:

Hello All,


Are there any ideas for a portable NVIS antenna for 75 meters that can be put up by one person on a flat parking lot without any high structures or light poles? 


Thank you,

--jeff

wb7aht

Bonnie KQ6XA
 

A loaded quarterwave vertical antenna (or mobile whip) with a single quarterwave radial, is an NVIS antenna. 

They are easy to put up and work well.

-Bonnie KQ6XA

felix1063@...
 

For those who might have access to EZNEC, Could somebody tell me what the center line impedance is for an 75 mtr dipole 30 inches (supported by tall traffic cones) above an asphalt parking lot?

If the impedance is way below 50 ohms, would a 4:1 balun be recommended to be used turned around so that the coax is connected to the high impedance side of the balun and the dipole elements connected to the low impedance side of the balun? I'd like to run this antenna without a tuner if possible.

Thank you,

--jeff wb7aht

Carson VA3OSO
 

I was about to disagree with you about the quarterwave being a NVIS antenna, then I thought about it.  If the antenna is loaded, making it electrically longer, then the radiation lobes will be going off the end of the antenna, making the vertical NVIS.  Thanks for making me think!  Learning something new everyday!

Carson
VA3OSO 



On Fri, Mar 3, 2017 at 1:11 AM, Bonnie KQ6XA <bonniekq6xa@...> wrote:

A loaded quarterwave vertical antenna (or mobile whip) with a single quarterwave radial, is an NVIS antenna. 

They are easy to put up and work well.

-Bonnie KQ6XA

Mike Saculla
 

Jeff,


EZNEC model of a copper wire dipole (#18 ga wire), 120 feet long, mounted 30 inches above average ground (don't know the ground parameters for asphalt), is resonant at 3.85 MHz and shows a SWR of 1.2:1 at that frequency.


Mike K6MDS


 
For those who might have access to EZNEC, Could somebody tell me what
the center line impedance is for an 75 mtr dipole 30 inches (supported
by tall traffic cones) above an asphalt parking lot?

--jeff  wb7aht


Peter Gottlieb
 

What impedance does the program show for it?


Peter

On Mar 3, 2017, at 1:27 PM, Mike Saculla <fqm@...> wrote:

Jeff,


EZNEC model of a copper wire dipole (#18 ga wire), 120 feet long, mounted 30 inches above average ground (don't know the ground parameters for asphalt), is resonant at 3.85 MHz and shows a SWR of 1.2:1 at that frequency.


Mike K6MDS


 
For those who might have access to EZNEC, Could somebody tell me what
the center line impedance is for an 75 mtr dipole 30 inches (supported
by tall traffic cones) above an asphalt parking lot?

--jeff  wb7aht


wb8apt@...
 

EZNEC results for feedpoint impedance of a 3.8 MHz resonant half-wave dipole 30 inches above ground:

Perfect ground, impedance 2.0 ohms.
Very good ground (conductivity 0.0303 S/m, dielectric constant 20), impedance 48 ohms.

Average ground (conductivity 0.005 S/m, dielectric constant 13), impedance 81 ohms.
Poor ground (conductivity 0.001 S/m, dielectric constant 5), impedance 109 ohms.

Using what might be reasonable numbers for asphalt (conductivity 0.001 S/m, dielectric constant 2.6), and presuming that these characteristics remain constant for the entire depth of relevant RF ground penetration, EZNEC says the dipole impedance is 120 ohms. The actual value likely depends on what else is below the asphalt.

Since these values suggest you might be in the ballpark with an SWR of 2 or less, you might just go ahead and try it as is. An antenna analyzer could easily measure the impedance directly after you set it up. 

--dave

Pete N
 

I have had very good luck with a dipole or G5RV as an inverted V. You can't get much simpler than that. I have a telescoping mast that puts the apex at about 19' tall. I made a drive-over base using a piece of plywood, a piece of PVC pipe, and a couple of shelf brackets. You could make a mast from sections of PVC. I don't think the height is all that critical. The only problem is that a 75 m dipole  or full size G5RV takes a lot of horizontal space. You could shorten the antenna by putting loading coils in it.

Andy
 

Carson wrote:

I was about to disagree with you about the quarterwave being a NVIS antenna, then I thought about it.  If the antenna is loaded, making it electrically longer, then the radiation lobes will be going off the end of the antenna, making the vertical NVIS.  Thanks for making me think!  Learning something new everyday!

Be prepared to learn again.

It's not the loaded vertical that makes it NVIS.  It is mostly that single radial.

The quarter wave radial is the other half of the antenna, and potentially radiates just as much as the quarter wave vertical does.  If you had a symmetrical arrangement of radials, their radiations would cancel one another.  But with only one, then the antenna is in an L shape with both a vertical radiator and a horizontal one.  Being asymmetrical, more of the radiation goes upwards, than if it had good symmetry.

Andy


wb8apt@...
 

Hi Jeff,

In addition to the info on EZNEC impedances over various ground types for your proposed half-wave dipole 30 inches above asphalt, I thought you might also like to see some far-field elevation patterns. The attached graph shows them for poor, average, and good grounds. (See my previous post for the conductivity and dielectric constant used for each one.) For reference, the strongest pattern in the graph is that of a resonant 75m half-wave dipole at 35 feet over average ground. 

One interesting thing about this model is that it suggests that these very low antennas will perform better over poor ground. I wonder if that's due to less loss.

--Dave

felix1063@...
 

Hi Dave and all,

Thank you for running the radiation patterns with different types of ground conditions. All this is leading up to a portable set up that can be handeled by 1 person in an empty parking lot with lots of traffic cones. The other option we have is a possible full block length planter which we may be able to use pvc sched 80 pipe with rebar as anchors to set up a dipole with the center at 7 feet and then ends at 10 feet.

More later.

--jeff
wb7aht

RLW
 

Hey Folks

I was wondering if a 75M application of my 40M 50W Emcomm speaker-wire short-sky-hook might work in the NVIS mode? My sky-hook is a (almost) full-band short -vert but I don't see why it might fly near-horz? Just wondering.


Mel Farrer
 

Put the diagram into the file section so we can view it better.  Thanks.


Mel, K6KBE

RLW
 

attached the jpg...

Mel Farrer
 

Left side border has all of the folders.  Got the jpg.


Mel, K6KBE

RLW
 

EmComm 40 Sky-hook construction notes:

1. Roll-out 100' zip-wire, fold in-half.
          (center fold-point is RF connector-end)
2. Flip-over one pair, starting from fold-point, so zip-wire pair will lay 'Red on Black on Red on Black'
          (as shone in cut-away view at connector)
3. Measure 2" from fold, mark and zip-tie that point. From that mark, measure 27' on each pair, mark-it.
          (now begins the "Yes, I am Anal" part)
4. Using PVC glue (I use Gorilla PVC type cause it washes-up with water) and a supply of don't stick them in your ears cotton-swabs (as disposable applicators) and large paper-clips (as clamps), lightly cement the pairs into a 4-conductor square 'Red on Black on Red on Black' for the entire 27' run as shone.

5. Zip-tie the squared-pairs at the end of the 27' mark. Un-zip the remaining 23' or-so pairs, red from black, resulting in four free 23' conductors.

6. Measure 7' from the fold-side zip-tie and mark-it. Fold-back onto its pair, to that 7' mark, each free-Black conductor. Remove the excess 3' or-so to create the Sky-hook's 1/4wave  'Matching Stubs'.  (33' x .6 v = ~20')

7. Again, using PVC glue, a supply of cotton-swabs and large paper-clips, lightly cement and clamp the free-Black conductors, centered between the red black pair on its side, creating the 6-conductor 20' run (as shone in the cut-away view).

8. Paralleling the remaining free 23' Red conductors, wind the loading-coil just after the 27' mark on a lightweight 2" form, 25 Turns. Zip-tie each coil end.

9. Attach the support line below the 27' zip-tie, feeding thru the coil-form and lace the remaining Red conductors paralleling to near their ends. (The line also provides strain relief)

10. At the fold, cut and prepare the Reds and Blacks for the connector of your choice. Both Reds to center, both Blacks to shield. After soldering, seal the plug with liquid-tape or similar goop.

11. Erect and tune, initially trimming both Red-lengths-jointly for best SWR on the your low-end of band, then trimming one of the Red's further for your band high end. Seal the ends with liquid-tape and zip-tie the run to the support line as needed.

Da-dahhhh.

Previous Topic Next Topic