Author Topic: Looking for suggestions for a directional antenna for circa 8 MHz  (Read 565 times)

Offline ChrisSmolinski

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There's a plethora of Brazilian pirates around 8 MHz, and I would like to try DXing them. They run fairly low power, so they are going to be tough catches. I've not had much luck yet with my existing antennas, other than the occasional carrier. Summer is approaching, and while it will not be the best time to hear them, it will be an opportunity to build an antenna, in preparation for the fall/winter DX season.

I'd like something with gain towards South America, or at least attenuation in other directions, since HF noise levels are usually well above receiver noise.  So... what to build?

A dipole is the easiest, and would give a few dB of gain.

That got me to thinking about two or more dipoles, spaced the appropriate distance apart, for additional gain. However the trees are not going to cooperate and be correctly spaced.

Next thought, an array of vertical antennas. I can space them as needed and even feed with appropriate coax lengths for proper phasing. But I'm a little leery of vertical antennas, with their reputation for being noise magnets. Plus I'll want a reasonable ground radial system for each.

That next got me to thinking about replacing each vertical with an amplified antenna, perhaps a small loop. But still placing them in a line as an array for directionality.  I am not sure how well matched all of the amplifiers would be in terms of preserving the phase of the received signal, for proper phasing for directionality. Other factors are the cost (lots of amps) and the question of how good reception will be, my experience with the loop I built with the LZ1AQ amp (which is a very good quality amp) is that signal quality is not as good as a full sized antenna. So several of these together may still not be great.

Any recommendations / other suggestions from the antenna gurus?

Chris Smolinski
Westminster, MD
eQSLs appreciated! Send to: csmolinski@blackcatsystems.com
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Offline Josh

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #1 on: April 15, 2018, 1814 UTC »
If you have the space and the trees, a wire beam does wonders. Just two elements and you'd be in high cotton. Linear or inductive loading could shorten it where you could put it on a tower as shortened dipoles. If you have any 40m mobile antennas around you might try those as a beam. If you made it switchable, asia may be in the pattern.

https://www.youtube.com/watch?v=cpxsMyoXUZQ&list=RDcpxsMyoXUZQ&t=5

Offline RobRich

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #2 on: April 15, 2018, 1852 UTC »
Have you considered tossing up a vertical wire for a quick check to see how much local noise is actually present?

I used to be concerned about vertically-polarized noise, but most of my antennas are vertically polarized these days. I am sandwiched on a suburban lot with one close neighbor and close power lines on two sides without too much noise concern, at least when then the grid is properly operating. I have had the power company out a couple of times to fix various issues.

Phasing verticals is straightforward enough, or even off-the-shelf with a basic MFJ noise phaser. Done right you would have a steerable array for many more directions. Use amplifiers at the vertical feedpoints, and you should not have to worry about massive ground radial fields. You could start with 4-8 radials each then add more as time and conditions permit - that is what I did for my 31' vertical.
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Offline Stretchyman

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #3 on: April 15, 2018, 2015 UTC »
Been said already, WIRE BEAM.

MOXON, is about the simplest/best, otherwise stick with Yagi with 3 or more elements.

Loads of other designs but Yagi is low profile and simple, you can load the elements as you won't be concerned with B/W.

Forget Verticals and amplifiers as they'll be too noisy, unless they're phased in a quadrant, somewhat complex however.

And Yes a proper mag loop would be sweet...

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« Last Edit: April 15, 2018, 2018 UTC by Stretchyman »
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Offline pinto vortando

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #4 on: April 15, 2018, 2158 UTC »
maybe a rhombic
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Offline ChrisSmolinski

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #5 on: April 16, 2018, 1050 UTC »
Thanks for all the comments, one additional thought is a beverage antenna. An advantage of this would be that I could use it on other frequencies, including MW, and it would be roughly perpendicular to my existing beverage that is aimed at Europe. I think I could get something about 300 ft long (the Euro beverage is 500 ft). Plus then I could feed the pair into the AFE822x for phasing. Hmm... whether or not this would work well for this purpose, it would be useful to do anyway.
Chris Smolinski
Westminster, MD
eQSLs appreciated! Send to: csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / 670 ft horizontal loop / 500 ft northeast beverage / 250 ft west-south-west beverage / 250 ft south beverage / 43mb sloping folded dipole / Crossed Parallel Loop

Offline ThaDood

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #6 on: April 16, 2018, 1632 UTC »
If you have the room on your property, you can't really go wrong with a bi-directional Delta Loop for HF,    http://w5sdc.net/delta_loop_for_hf.htm

Again, another passing thought.
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Offline pinto vortando

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #7 on: April 16, 2018, 2315 UTC »
The Beverage is mostly used as a lower frequency antenna, 80 meters generally considered the upper limit.  Hence my thought about a rhombic.
However, if you have the real estate, give the Beverage a try.  If it doesn't work out at 8 megs no big deal as you still could use it for LW/MW stuff.   
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Offline refmo

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #8 on: April 17, 2018, 0011 UTC »
I agree with the rhombic, or its cousin the Longwire Vee, depending on available supports. 3 needed for the Vee, 4 needed for the rhombic.
I used to have a longwire vee, best antenna I have ever used.  I could regularly work SE Asia on 40 meters, as well as long-path to Australia from Kentucky.
300 feet would have fair gain (400 feet would be good).  The higher the better but it isn't as sensitive to height for take-off angle as a dipole.
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Offline redhat

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #9 on: April 17, 2018, 0453 UTC »
Our own Kilokat7 has had good luck with the Dkaz Loop

http://www.durenberger.com/documents/DKAZREPORT.pdf

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Offline CaptnKliegle

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #10 on: April 17, 2018, 1114 UTC »
At field day, we have put up a 3 element wire 40M beam. All elements are inverted Vee's and supported by one rope held tight. From our location in NH, 99% of all FD QSO's are SW of us, so we run it pointing SW. It's a nightmare of mason's twine holding the element ends up, but for the gain, well worth it.

Our FD team has taken 1st place is 5A numerous times over the years.

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Offline ChrisSmolinski

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #11 on: April 17, 2018, 1152 UTC »
Our own Kilokat7 has had good luck with the Dkaz Loop

http://www.durenberger.com/documents/DKAZREPORT.pdf

+-RH

Aha yes, I had forgotten about his experiences with the DKaz loop. I wonder how it would scale down in size for use around 8 MHz. If linear (or close), it would be a fairly modest sized antenna.
Chris Smolinski
Westminster, MD
eQSLs appreciated! Send to: csmolinski@blackcatsystems.com
NRD 545 / netSDR / AFE822x / AirSpy HF+ / 670 ft horizontal loop / 500 ft northeast beverage / 250 ft west-south-west beverage / 250 ft south beverage / 43mb sloping folded dipole / Crossed Parallel Loop

Offline RobRich

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #12 on: April 17, 2018, 2231 UTC »
If considering a beverage, then a grasswire (unterminated beverage-on-ground) might be another idea to toss on the consideration list.

http://f5ad.free.fr/Liens_coupes_ANT/G/K3MT%20Antenne%20gazon.htm

Note the peak takeoff angle is between 10 to 25 degrees. Matching is done with a simple trifilar balun.

I used to run a similar antenna for DX listening to the southwest with good results.
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Offline ThElectriCat

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #13 on: April 25, 2018, 1509 UTC »
I would also tend to say beverage, but unfortunately not one that would work at lower frequencies well, I built one a while back for the 40 meter ham band, and it was one wavelength and about 4 feet off the ground. The termination will probably be more touchy than a longer one, but once it was terminated right it worked quite well. I have never tried a beverage that small any higher off the ground, and I imagine that its performance would pretty quickly change near 1/4 wave up
wj DMS-105a-2, and whatever wire, loop, or other antenna I can get my hands on

Offline Josh

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Re: Looking for suggestions for a directional antenna for circa 8 MHz
« Reply #14 on: April 25, 2018, 1747 UTC »
Ooooh yeah a rhombic! With the reentrant termination developed by the enguneers at VOA! Reentrant as they fed the sig back to the feedpoint via a phasing line so no need for a power wasting load as found in every other rhomby. However, I fear a rhombic would please a wife less well than the other candidates mentioned.