Mounting Antenna for NVIS vs DX operation ?

[solaris0121]

Hi guys, my 1st post here as newly licenced ham in the UK (M6OLR).  I have a couple of PRC-320 radios ive had for a while and with the new licence i installed a 5band EFHW antenna from the chimney to a pole down the garden.

I noticed on 20m and 40m i dont really pick up any local stations in the UK and I understand to do this I need to mount the antenna for NVIS use.  As I understand it, is this simply a case of LOWERING the antenna and ensuring its horizontal ? *eg to fence height, a metre or so off the floor. 

Im thinking that when im out and about portable i will get a 3 band EFHW and use a fishing pole with it mounted vertically for DX and horizontal (low down) for NVIS - or is it more complex than this ?  Tactically, I like the option to change my radiation pattern when out in the field portable.

Interested to hear your thoughts...... M6OLR

[gil]

FB on the 320, I love mine!

Not surprising on 20m, but you should get regional contacts on 40. 20m just doesn't work for NVIS, which will work only with 40/80m. Higher frequencies will just shoot up into space and be picked up by aliens in thousands of years  :o Horizontal is better for NVIS but sloped should work too. Vertical is for long distances, and that would be best on 20m. So, go horizontal on 40/80m and vertical on 20. You don't want to lower your antenna too much, 20ft is fine.

For regional/local contacts, 80m is your best bet. We would have no problem making 320-to-320 contacts on 80. Oh, wait! Let's try!

Gil.

[solaris0121]

Thanks Gil, i was hoping that lowering a short EFHW almost horizontal to the ground (a few feet , tied to stakes or a tree) might force it into operating NVIS  on 40 and 20.....  the reality is I should just get out there and try it...... im just a little new to all the theory.

I have 2 of the 320's, one of them is the thick finned version.  Ive just purchased some capacitors for the power supply modification which I need to do very soon.

The past few days since the antenna went up ive been calling out on 7187.5 (prc320) as well as 14348 and 3793.   Are you guys using these frequencies ?  Would be great to have a 320 to 320 QSO over the channel !!!     73's Sean.

[gil]

I was hoping that lowering a short EFHW almost horizontal to the ground (a few feet , tied to stakes or a tree) might force it into operating NVIS  on 40 and 20

Unfortunately not. I would go no lower than about 7ft off the ground for 40m. NVIS is just not going to work on 20m. You might get regional contacts on 20 sometimes, but it will be due to E-layer propagation, not NVIS. Too close to the ground and you will have too much ground losses. It depends on your ground of course... Laying a dipole on desert sand might work very well! If you try a wire antenna 7ft off thre ground on 40m, try laying a 5% longer wire beneath it on the ground, not connected to anything, as a reflector... You might actually use three wires. See my post on the "Super Gain" NVIS antenna:

http://radiopreppers.com/index.php?topic=1260.msg11820#msg11820

My 320 is a thick-fin model too. I should not have sold my other one... I also have a PRC-351, and a 352.

You would have better luck using USB (since the 320 doesn't have LSB!) on 3615 (3617 on your dials), especially Wednesday nights for the VMARS net. I haven't participated in a while, but really should get back on...

We can try any weeknight on 80m, let me know...

Gil.

[Andywragg]

Hi and Welcome.

I'm in IO93 (South East Sheffield) I tried my NVIS  X-Pole (2 inverted V dipoles at 90 deg to each other) last weekend and didn't get 1 contact all afternoon. I'm using an old military design for the antenna with the centre/balun at 15feet AGL. The dipole wires are cut for 25' (40m) and 37' (80m), 4 wires in total. The radio is a Yaesu FT-817. The trouble with NVIS is that any receiving station also has to be working NVIS propagation to pick you up - or so I've been led to believe. 

Maybe we could work on a sched to try and establish a net. The trouble with 40m is that it's very popular with contesters at weekends and trying to find a clear frequency is a challenge to say the least. I found 80m to be really quiet from a noise/QRN POV.

[Andywragg]

This is the design I'm using

https://static.dxengineering.com/global/images/instructions/WP-NVIS-Rev3.pdf

[solaris0121]

Nice link Andy, something g like that could be made up, rolled around your support mast and be slung in the car, ready made to guy out.....

I wonder what it's also like for dx when pushed up higher - and the wires are more vertical ?

[gil]

The trouble with NVIS is that any receiving station also has to be working NVIS propagation to pick you up - or so I've been led to believe.

No, they will pick your signal, but you might not receive theirs...

Furthermore... Most horizontal dipoles are not high enough to give you a low angle of radiation, which means that most dipoles will be good for NVIS but not DX. Verticals on the other hand will be good for DX but not NVIS. So, on 80 and 40, use a dipole for NVIS. On 40m and up, use a vertical for DX. You can get good DX with a dipole on the higher bands, but it has to be high enough, half a wavelength.

Gil.

[solaris0121]

I came across the "grasstenna" - a regular long wire or dipole laid out on the grass...... the drier the ground is apparently the lower the losses.  Sorry I cant find the link now, but it may be worth testing...... probably similar to your video Gil where you had your wire laid over the bridge.  Sean.

[ron_c]

Here's a good article on building a NVIS antenna, but this is the part that a lot of folks miss reading.

"NVIS only works at frequencies from 2 MHz
to 10 MHz. The signal must penetrate the D
layer of the ionosphere, and bounce off the F
layer. Lower-frequency signals will not
penetrate the D layer; higher frequencies will
not bounce off the F layer at these sharp angles and just goes out into space. Remember the
Maximum Useable Frequency (MUF? For amateur radio operators, we're looking at 40 and 80
meters primarily for NVIS use."

http://arrl-ohio.org/SEC/WP-NVIS-Rev3.pdf

[Andywragg]

Nice link Andy, something g like that could be made up, rolled around your support mast and be slung in the car, ready made to guy out.....

I wonder what it's also like for dx when pushed up higher - and the wires are more vertical ?

This is me testing it out the other weekend.


https://youtu.be/ZsOppHCRH08

[gil]

didn't get 1 contact all afternoon. I'm using an old military design

It's probably not the greatest idea to copy the military design, as far as wire lengths are concerned. They are not optimized for the ham bands. Better use half-waves of the bands you want to use. Also, the evening is much better for NVIS on 40 and 80m, after sunset.

Gil.

[Andywragg]

Hi Gil,

The design I'm using is a Ham design, based on the military version. Its actually from DXEngineering (see my link earlier). To be fair I was only there, set up  for about an hour before it started to rain, so chances are with a bit more time on station I would have probably got a contact.

The FT-817 that I had on loan has now gone back to the owner and he's traded it for an FT-897 so my /P activities are somewhat curtailed until I can get my uBITX finished, and I'm still waiting for the case to arrive, or even be shipped. Alternatively I could go /P with the FT-450 and operate from the car but that would mean dismantling the shack.

[gil]

That uBitx will serve you well. Keep us posted!

Gil

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