Antennas for Beacons (tricky terrain)

[Zazzle]

Hi everyone!

I have two primary issues that I'd like to address. One regards an actual problem and the second is a theoretical one for an upcomming project.

Actual Problem:
It regards this project.
I put a 1,5W Beacon up on a roof. 10.240MHz. It looks like it doesn't radiate the field properly. Maybe the issue clears itself on it's own today evening when I check the correct lengths for the Dipole. I may have cut 'em too long. But I also suspect other issues with the location in general as well. As you can see on the attached photos, the Transmitter is installed on a roof with lots of metal constructions around. Given the low Frequency I suspect the construction to interfer with my antenna. Actually, my questions are:

• How much may the roof construction interfer with my antenna?
• I suspect that "as high as possible" is the key for the antenna? I also read that a height of 1x wavelenght is perfect for Dipoles?
• Does an Inverted V, say, with a 90° angle help here?
• The Dipoles are constructed as follows: Dipole wire - 1m of Nylon wire as insulator - (conductive) wire/cable until mountpoint on the ground. Should I use a Nylon wire for the entire length for the support?
• I live in a really, really big city but close to the city limits. How much may the city and it's interferences interfer with my signal?
• Should I connect the GROUND from my circuit to the roof construction?

Theoretical issue:
I'm going to build a second Beacon. Around 7MHz this time. I like to install it on an old air field.
Some fact on the air field:

• No building around for at least 300m.
• The ground exist of sand and ist mostly dry during summer.
• Little vegetation. A bit of grass and some random trees.

I think it's save to erect a wooden slat of about 4m as the center support for an Inverted V without no one taking notice of it from far away.

So, my questions are:

• Is there an Antenna type you'd suggest? I'd go for an Inverted V, even if the angle is kinda low.
• Maybe a "KGD" or a Micro Vert works better?
• Does it make sense at all? Is a high mounting point for the Antenna the key and I can spare myself the effort anyway?

Kind regards,
~Zazzle

[Zazzle]

*chuckles* Well, looks like I have to find the answers to my questions on my own. I'll let you know about the outcome.

[Antennae]

Hello Zazzle.
-With antennas, metal interferes with dipoles. Maybe it would be better to use one wire for positive and then connect your antenna ground to the metal like you said.
-High is good in general.
-Inverted v is like a dipole.  A dipole close to the ground gives better close range coverage (research "NVIS") than a dipole high up which gives better long range.
-Nylon wire for the entire length would be better than what you have described.
-I think that if you are trying to listen for your beacon and you're in the city, if you could get about 50meters from a building, light pole, and electric wires, you would reduce a lot of interference and would have a good chance at hearing it.

On the air field, a dipole is the most efficient use of your power. And since it can't be mounted very high it will be giving good close range coverage (which is hundreds of miles in radius).  If I were you, I would look at the wooden slat from different distances and positions and then paint it to blend in with its background.  It could be as simple as leaving the bottom half brown but paint the top half a lighter brown or grey color.  Or fade it into the lighter color so it would blend with the sky. 

There's a beacon called the Inyo Whooper. I hear it and I'm a couple hundred miles from it. Its wires don't look very high So maybe yours would have the same characteristic.  http://www.auroralchorus.com/4096khz1.htm

An important thing to think about with an antenna is its impedance.  Its a complicated subject. For the PERFECT antenna: The dipole elements come together and that junction has an impedance measured in ohms.  It needs to match the impedance of the feedline that goes to the transmitter.  And then the impedance of the feedline needs to match the impedance of your transmitter.  Baluns match elements to feedlines.  And similarly, antenna tuners match feedline impedance to transmitter impedance. 
-BUT your antenna is close to the ground and the impedance probably wont match your feedline or your transmitter. It would work but who knows how far.  But with good propagation it could go thousands of miles, like the Inyo Whooper has.

[Zazzle]

Hi there Antennae!

First of all: sorry for the late reply. Especially after my previous comment. That's a bit embarrassing.

And, thank you for your elaborated answer. I should have said that I'm not completely new to Antenna theory. The issue is that I lack of practical experiences for Antennas in frequencies that low in the current situation. I've been active on UHF/VHF before. Also, I've read that the real thing behaves sometimes completely different from the theory. That's whay I asked. I.e. I read a lot in "Rothammels Antennenbuch". With may be the European equivalent to the ARRL Antenna Book.

Antenna-Type: I'll try a different Antennas during the next days. The German word is "Fuchskreis". No idea what the proper english Term is. See attachment. It's a single pole, voltage feed (high impedance) Antenna that's feed from a parallel resonant circuit. The key feature is the tunable circuit which allows good matching. Also, according to the Antenna book, the impedance stays around 2500R in a wide range of applications and situations. That may help.

But to get behind the issue - whether I have a Impedance matching or location problem - I'll separate Transmitter and Antenna in Future which will allow mw to loop a SWR/Wattmeter into the line for reading.

I asked about the Antenna height because there's a chart for the regular Dipole which shows that the impedance near ground (in regard to a certrain fraction of the wave length) drops around 50R. But yeah... quite possible that the metal construction interfers with the field pretty bad.

(Btw: about the setup at home: NRD-525 / FT897D / PSR-225 / 40m Dipole with Balun and tuner / End-point feed dipole with tuner. So I guess it must be an issue on the TX side)

Sadly, I haven't much options for other locations. The old industry yard is the best suitable so far. But maybe I can have the Antenna run into a different direction - away from the roof construction without it being too visible. I'll just try that in case the Fuchskreis doesn't help. Thank you.

About impedance matching: I use a 5-Pole PI-Filter for impedance matching (PA-Transistor and output) and harmonics supression. It matches the output impedance to 50R. I asked about the BalUn becuse I read that Dipole Antennas like to be feed with a symmetric signal. And the output of my Transmitter isn't. But yeah. All theory ends when the Antenna joins the game. Anyway, as soon as I'm able to loop a meter in I'll know more about how it behaves in reality.

Thank you for the time and advise. I hope it'll improve the project. And I keep you tuned.

Kind regards,
~Zazzle

[Pigmeat]

I knew a fellow who ran one of those beacons in the southwestern USA. His antennas were half-wave longwires, plugged directly into the antenna jack via a banana plug. He strung the antennas through the local brush at about 8 feet above the ground.

I could hear his 150 mW beacon regularly in the late afternoon, in the freq. range around 8000 khz., at a distance of nearly 2500 miles.

Don't sweat the antennas. Try a few that you think will fit your needs, see what works best, and go with that design. You can experiment with other types after you have a functional antenna to compare them with.

[Zazzle]

Hi again,

After having a lot of further frustration with this project, I decided to put it aside for some weeks. Sometimes a little distance helps to obtain good ideas and having a look at the issue from another perspective.

So, what did I do?

• I checked the wire length of the Dipole. Yep, it was approx 1,5m too long on each side. I corrected that but the received signal strength didn't improve (at my home)
• That made try it with a BalUn. After testing several impedances, I eventually settled on a 1:2 BalUn (50R:100R). Which gave me the best SWR of about 1.4 in my backyard (with the Antenna about 2m above the ground).
• I put the Antenna on the industrial roof again and even higher this time. About 3,5m above the roof. Sadly, no big change. The SWR read 1.2, which is fine.
• Sigh! I took the whole setup back home and installed it on my own roof. Which is 8m above the ground and aditional 2m for the pole where the transmitter is installed on. Again.... no big change. What a frustration! The signal strength behaves like on the industrial roof and I have a hard time picking it up beyond 1000m around the transmitter.

What I'll do next:

• Testing the Transmitter with a different Antenna. I have an old unused CB Antenna flying around and I'll try to match it with a Antenna Tuner
• Actually, I start to believe that the Antenna isn't the problem but maybe the filter. (I started a diffent topic about it.) I mean... it delivers ~2W into a 50R dummy load but maybe isn't able to deliver it into the Antenna... problems with reactive load... or what ever. No clue.
• I'll take the Setup along on a vacation (mountains). Maybe the setup behaves different outside of the city. I doubt it but time will tell.

So, and other suggestions for possible problems?

Greetings,
~Zazzle

Oh, I also painted the pole. That was a good suggestion. Blends in with the background way better. Thanks.

[Pigmeat]

Hmmmm..... With a low power tx and near vertical incidence antenna like you're using, you may simply be outside the range of the groundwave of the antenna and in the skip zone at a kilometer away

A NVIS antenna sends a signal nearly straight up, and it reflects back down relatively close, but not too close. I've had reports from 25 to 600 kilometers away during a single broadcast using an NVIS antenna.

Do a search for online receivers 150 to 300 Km's from you. Check for the signal there the next time you set the beacon up. You may be pleasantly surprised.

[ChrisSmolinski]

I agree with Pigmeat's comments regarding a possible skip zone around your site, due to NVIS. Solar conditions have not been very high, resulting in lower foF2 frequencies (that is the highest frequency that will be reflected straight back down).  There's a real time map of oF2 frequencies here: http://www.spacew.com/www/fof2.html

Depending on the current conditions when you're doing your tests (which varies with the time of day and solar conditions), foF2 could be well below your 10.24 MHz frequency. This means that your signal will not be reflected back down to near your transmitter site. You might have to travel tens or even hundreds of km away to hear it.

I posted a bit about NVIS here, and in some other articles on the same site: http://www.radiohobbyist.org/blog/?p=245