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Author Topic: Short Distance HF  (Read 110521 times)

Posts: 27

« on: August 31, 2014, 08:34:53 AM »

I need to be able to communicate to the EOC from 26 miles out. I can't reach a repeater very well so I was thinking of using HF (the EOC has an HF setup). Now I know that some hams use NVIS and skywave for short haul communications. My setup is a vertical. The question is the best frequency to attempt to make contact using the vertical. 80/160 meters for ground wave, or 10/6 for direct, or something in between. We are going to try the experiment eventually but it would be nice to know where to start.

Posts: 35

« Reply #1 on: August 31, 2014, 09:24:53 AM »

I have used 6M FM fairly consistently from that range, but there are a lot of factors involved.

I worked comms for several endurance races in remote wilderness areas so I'm familiar with the issue, but we went a different route and built portable repeaters that we set up on mountain tops in the area. They were built simply, in a pelican case, output 50W, and ran off an AGM battery with a solar panel. We could operate 48+ hours without interruption.

Posts: 27

« Reply #2 on: August 31, 2014, 09:52:31 AM »

I do that as well. I have a 50w 2m/440 that I use with a deep cycle marine battery and a vertical on a portable mast (next use Sept. 14th). I manage in excess of 50 miles to the repeater. EOC has (in addition to 2m) a fixed HF station. Hills between me and the station preclude 2m. So I am trying to figure out what can be done on HF.

Posts: 17418

« Reply #3 on: August 31, 2014, 10:35:54 AM »

NVIS likely would be a better option, even if the antenna is just laying across
some low bushes.

Ground wave coverage depends on your local ground conditions, output power,
receive noise level, antenna efficiency, and antenna polarization at the other end.
I think the ARRL charts suggest a maximum coverage distance for ground wave of
about 50 miles on 80m with 1kW over average soil, half that on 40m and double
on 160m. Edgewood, however, is not blessed with such good soil conductivity.
That also assumes vertical polarization at both ends:  perhaps the important
first question to resolve is the antenna polarization at the EOC, then plan your
approach to match that.

With efficient vertically polarized antennas at both ends, 160m will give the best
ground wave coverage, and because it doesn't depend on the ionosphere it should
be more reliable over time.  But efficient verticals are not the norm on 160m,
and receive noise at the EOC may be a limiting factor. Note also that the typical
distances assume 1 kW transmit power at both stations.  Ground wave has a high
rate of absorption, so the limiting factor is where they get too weak.

By contrast NVIS has relatively low attenuation when the ionosphere cooperates,
and low power can work well.  If you have enough antenna height for an efficient
vertical, you have plenty for an NVIS dipole.  But you need to keep an eye on the
propagation and change bands (160m, 80m, 60m, 40m) as conditions require.

Ground wave coverage on 10m / VHF is very limited, but direct propagation
(space wave) can cover those distances with enough height at each end to clear
obstructions. From Edgewood it looks like you have a better chance of reaching Santa Fe
than Albuquerque, even though the latter is closer, because of the terrain.  With
50 watts on 2m you may be able to bounce a signal off one the mountains to your
NE and down into Santa Fe with a big enough beam.  6m is certainly worth trying,
especially with a beam of some sort, as long as your antenna is at least roof high.

And you may find that SSB gives you much better VHF coverage than FM.

For a 100w station, a low dipole likely will be better than ground wave as long
as you are operating below the critical frequency.  But VHF is worth trying,
depending on the details of the specific path, especially using beam antennas
at a reasonable height above ground.

I would ask the locals first - they will have a better experience of typical coverage
in the local conditions.

Posts: 244

« Reply #4 on: August 31, 2014, 12:43:21 PM »

We've experimented with this.  Make a plain dipole or OCFD for 40 meters, about 10' off the ground.

40 meter is your best bet during the day in well into the evening.

Posts: 8

« Reply #5 on: August 31, 2014, 01:43:13 PM »

I agree with WB6BYU - vital factors to consider are the noise level at the EOC and the type of antenna(s) installed there. You want to use your vertical antenna? That pretty much rules out NVIS. If the noise levels at the EOC and your location permit, groundwave on 80 or 160 might be okay. If the EOC has a horizontal antenna, I would recommend you put up a horizontal wire and try NVIS on 160 or 80 meters. Keep in mind, though, summer static can make SSB miserable to use for long periods of time. If noise is a problem on MF/HF, try 6 meter FM. Depending on the path, that might work out better. VHF bounces around the mountains surprisingly well, although path prediction is not always accurate. You just have to try it. An advantage of 160, 60, and VHF is that you can switch between voice and digital modes on the same frequency.
Given your stated preference to use a vertical antenna and HF, I'd recommend 160 or 80/75 meters, with the caution that you need good radial systems at both your location and the EOC to enhance your groundwave signals as much as possible. Get some good DSP noise reduction hardware, or consider going to non-voice modes such as MT63 or MFSK so you don't have to listen to the static crashes. Hope this helps. 

Posts: 12492

« Reply #6 on: August 31, 2014, 02:32:15 PM »

Have you considered 10m FM?

Ham since 1969....  Old School 20wpm REAL Extra Class..

Posts: 27

« Reply #7 on: August 31, 2014, 02:50:27 PM »

Not sure what the EOC has for a radial system. I suspect they are using a GR5V (no radials required). My vertical system uses 26 radials of 33' each. Definitely what I believe is a decent radial system :-)

When we do the experiment we will try both SSB and FM on 10m. I have had good luck with 10m FM on level ground even with a high noise level. Too bad 11m wasn't FM :-)

Posts: 17418

« Reply #8 on: August 31, 2014, 03:30:00 PM »

Quote from: K6JHU

Not sure what the EOC has for a radial system. I suspect they are using a GR5V (no radials required).

That will be horizontally polarized most likely, so it won't work well for ground wave coverage.

It is possible to adapt it for vertical polarization as a "T" antenna by shorting the twinlead at the
base and feeding it against ground (which does require radials.)  But since all the other stations
they expect to work will be using NVIS, this might not be practical operationally.


 My vertical system uses 26 radials of 33' each. Definitely what I believe is a decent radial system :-)

How high is the vertical antenna you are using?  How is it loaded on the lower bands? The
question is not just the radial system but the overall antenna efficiency.  For 80m and
especially 160m you're going to need a pretty tall vertical to manage good efficiency, and
you still have to deal with the low conductivity soil along the desired path.  (Radials won't
help that, unless you can run them for 26 miles...)

A horizontal dipole at 20' will provide much better signals under most conditions.

Posts: 150

« Reply #9 on: September 01, 2014, 08:02:57 AM »

6m if you have the path. Might also try 12 and 15

Posts: 479

« Reply #10 on: September 01, 2014, 08:31:55 AM »

For 80 meters, I have an HF2V vertical, AND currently experimenting with a very low 80 meter dipole - only about 15 - 20 feet up in the center.

The vertical certainly works better for longer distances more than 400 miles and does work for close-in stations out to about 30 - 40 miles. But that leaves a big gap. That's why I am trying the dipole - to fill in that 400 mile gap. And it does that quite well.

Also at that height, it's real easy to put it up since I only need to get a rope over low branches in the trees and there's little chance of getting the wire tangled up with branches. It zig-zags and is supported at four points. Just because that's where the trees are. It's the sort of antenna that one could have coiled up in a bag, then pull out and string it up in less than 30 minutes for an emergency when needed.


Posts: 529

« Reply #11 on: September 01, 2014, 10:09:44 AM »

10M band sounds perfect for this.

Posts: 27

« Reply #12 on: September 01, 2014, 10:14:29 AM »

10m was my best guess. And FM as well. Can't do 160. Can only get as low as 80. So 80 SSB was my best guess second choice. Still open to suggestions.

Posts: 17418

« Reply #13 on: September 01, 2014, 03:05:56 PM »

Quote from: KC2UGV

10M band sounds perfect for this.

Why do you think 10m would work given the path involved?

Assuming you are trying to talk to Santa Fe from Edgewood the path is actually fairly open without
major blocking hills (as opposed to the path to Albuquerque with mountains that rise another 3000'
in the way.)  In fact, I'm surprised you can't hit the repeaters on Sandia Peak and/or those on the
hills outside of Santa Fe.

10m has very limited ground wave coverage - a few miles at best.  The direct wave might cover
the path if the antenna is high enough and/or enough antenna gain, but in that respect it is no
different than any of the other VHF bands (except that gain antennas are more convenient on
6m and/or 2m.)  Height is important to clear the local obstructions:  getting an antenna up to
30' or so will probably make a significant improvement on any of the bands.  The combination of
that and a beam with a boom length of 6' to 12' or so may be sufficient to cover the path, at
least on SSB.

One way to verify this would be to take a quick drive to the top of Thunder Mountain, a quarter
mile or so to your West.  See if you can hit any repeaters with a good mobile antenna from up
on Snowflake Trail that you can't hit from your house.  One reason I'm recommending repeaters
here is that they will be elevated at the far end, which makes the path much easier.

FM is a strong signal mode - it doesn't work well for weak/marginal signals.  SSB, CW and some
of the digital modes are much better when signals are weak.

One thing to remember with NVIS propagation is that conditions change throughout the day.
At this point in the sunspot cycle it may be that 80m is better at night and 40m during the day,
though here in Oregon at the bottom of the recent cycles we have needed to use 80m during
the day and 160m at night to maintain coverage.  You can run forecasts for the expected
coverage areas for each band using the propagation tools on the Australian Ionospheric Propagation
Service here:  The "LAMP" charts ("Local Area Mobile
Prediction") are the most convenient ones I've found for forecasting NVIS coverage out to
several hundred miles.  (And the tools have an option that has the ham bands already built in.)

Posts: 27

« Reply #14 on: September 01, 2014, 06:21:00 PM »

Trying to get from Edgewood to Albuquerque. Can't hit Cedro peak from here. Can only get to the 2m repeater in Clines Corners. That is why I would like to use HF.
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