Specialist passive utility antenna

danelaw

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May 15, 2020
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Hello to all on the forum

There is a lot of information on the internet reference resonant and non-resonant wire antennas and their lengths, feedpoint transformers etc. But nearly all this information is aimed specifically at the amateur radio user and their hf bands

As a utility monitor, I am trying to gather as much information as possible to build a passive antenna which is best suited for monitoring the utility bands.

My areas of interest are the hf marine and aviation bands especially the 4, 5 and 8MHz bands.

I already have a Wellbrook ALA100 and it serves its purpose, but I'm looking for a passive alternative to try at my QTH which is fairly quiet.

The ideas I have are:

33ft vertical fed by an appropriate transformer (9:1,16:1, 32:1)

End fed inverted-L at a given length that is suitable for these bands fed with either a 9:1, 49:1 or 64:1 transformer

Unfortunately, a dipole is not suitable at my location due structural issues.

One of my main questions are the specific length of the end fed inverted-L for the utility bands. Do I go for length that is resonant on the ham bands to make it non-resonant on the utility bands?

And what ratio transformer would be suitable for the 33ft vertical?

Your help is very much appreciated
 

ab3a

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Oct 8, 2007
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328
Location
Lisbon MD
Since you're monitoring, you don't have to use a resonant antenna. The name of the game is signal to noise ratio.

If you scratch around for technical information you'll see lots of nearly religious arguments on which is better, a loop or a dipole/vertical antenna. The truth is that for your purposes, it doesn't matter that much. I'll give you a hint: your house and the houses around you are filled with electrical noise that won't do your signal to noise ratio any favors.

The favorite small antenna that many use is a loop. Loops are good antennas because they can be balanced and isolated from your coaxial feed line. Most receivers aren't that sensitive, so many people will use a loop with an integral preamplifier to boost the signal strength. If your receiver is of a recent design (say over the last several years), then you're probably better off without the preamp. If you do opt for a preamp, don't get a cheap one. It will distort the weak signals with the strong ones.

If you want to use an inverted L antenna or a short vertical, consider decoupling (isolating) the antenna from your feedline going in to your house. A 9:1 UnUn transformer can help. Plug the keyword UnUn in to a search engine and you'll see all sorts of stuff.

Here's a sample: 9:1 Magnetic Longwire Balun / Unun – M0UKD – Amateur Radio Blog

This is not difficult to build, and it should perform reasonably well.

There are other options as well. Nevertheless, for receiving, efficiency of the antenna is secondary to signal to noise ratio. Don't worry about resonance, worry instead about getting away from the RF noise in your home.
 

Ubbe

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Sep 8, 2006
Messages
4,486
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Stockholm, Sweden
Just last week I put up a 20 meter long wire and used a 9:1 long wire balun and then RG6 coax to my house. I connected to the same coax as is used to the satellite dish using a $10 diplexer. It didn't matter to the signal when I cut it from 21 meters to 20 meters. What does matter are the weather and the time of day that will make signals dissapear totally and later appear at full strengt in the whole band depending how the signals travels thru air.

One good reference are the airplane information channels that transmits continously on all different shortwave bands. But remember to be quick with modifications and monitoring as conditions shifts very quick.

I got those $10 telescope fishing poles that's 5 meters long and hoisted the wire up in the air to get some distance to ground.

/Ubbe
 

ka3jjz

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Location
Bowie, Md.
When talking about antennas at HF, in this case, forget the service. Broadband antennas like end feds don't care about the service you're trying to hear, but the frequency does matter.

There are several companies such as MyAntennas that are selling end fed half wave antennas. I've never tried one myself, but it ought to work just fine for what you want to do. Of course, if we're talking end feds, there's one very famous one that is quite popular. You can change the antenna wire out for whatever length you want, and you will end up fooling around with the connections on the box to find the quietest one for your location. Every setting is different, RF wise.


If you're short on space, I'm told this design is very popular in Europe..


Mike
 
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prcguy

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Jun 30, 2006
Messages
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So Cal - Richardson, TX - Tewksbury, MA
This is all good info from ab3a. If you look at antenna lengths vs impedance you will find for a wire run horizontal above ground at a reasonable height, the impedance for a wire 1/4 wavelength at the lowest frequency you will use will vary from maybe 60 ohms to over 2,500 ohms as you go higher in frequency. At no point will the impedance go much lower than 60 ohms unless you use a frequency where the wire is shorter than 1/4 wavelength, then it can go down to 10 ohms or maybe lower at some frequencies.

As a compromise its common to use a 9:1 transformer to smooth out the impedance fluctuations when feeding the wire with 50 ohm coax, which matches to about 450 ohms and is not a bad place to be over a large frequency and impedance range. I have also seen a 5:1 transformer used by some companies and that matches to about 250 ohms. The worst case would be when the antenna is exactly 1/2 wavelength long where you will see the highest impedance at 2,000 or 3,000 ohms and more of the voltage picked up by the wire will be lost in the mismatch.

A 49:1 or 64:1 transformer would be used with a resonant half wave or multiples of a half wave of wire, which is sharply tuned and these transformers are not the best for broad band use. They would be trying to match the 50 ohm coax to 2,500 ohm or more and the antenna will be much lower impedance than that over most of your receiving range.

A vertical antenna will pick up more noise than a horizontal antenna for several reasons. A lot of noise is vertically polarized and the vertical antenna will have a lower angle of takeoff picking up distant thunder storms, etc. A horizontal wire will have a higher takeoff angle unless its waaaay up there and will generally giver a lower noise floor. There are other things to consider like a wire antenna close to or over your house might pick up more RFI from inside the house and be worse off than a vertical way out in the yard away from stuff.

For matching a vertical a 9:1 or 5:1 should be fine and a 25ft to 43ft or similar should work ok. I have a commercial/military vertical receive only antenna made by TMC (Technical Material Corporation) commonly used at US Embassies that is an 18ft whip with a 9:1 transformer in a box and these were used at ground level and sometimes on the roofs of buildings to supplement the Embassy receive capabilities. They seem to work ok.

The bottom line with HF reception is signal to noise ratio and when comparing 25ft of wire or 50ft or 100ft connected to a 9:1 transformer, in most cases the signal to noise ratio is about the same and the longer wire gives you more signal AND more noise but the SNR does not change. Going to a vertical might pick up distant signals better but the SNR could be worse due to higher noise. Yada yada, etc.

Since you're monitoring, you don't have to use a resonant antenna. The name of the game is signal to noise ratio.

If you scratch around for technical information you'll see lots of nearly religious arguments on which is better, a loop or a dipole/vertical antenna. The truth is that for your purposes, it doesn't matter that much. I'll give you a hint: your house and the houses around you are filled with electrical noise that won't do your signal to noise ratio any favors.

The favorite small antenna that many use is a loop. Loops are good antennas because they can be balanced and isolated from your coaxial feed line. Most receivers aren't that sensitive, so many people will use a loop with an integral preamplifier to boost the signal strength. If your receiver is of a recent design (say over the last several years), then you're probably better off without the preamp. If you do opt for a preamp, don't get a cheap one. It will distort the weak signals with the strong ones.

If you want to use an inverted L antenna or a short vertical, consider decoupling (isolating) the antenna from your feedline going in to your house. A 9:1 UnUn transformer can help. Plug the keyword UnUn in to a search engine and you'll see all sorts of stuff.

Here's a sample: 9:1 Magnetic Longwire Balun / Unun – M0UKD – Amateur Radio Blog

This is not difficult to build, and it should perform reasonably well.

There are other options as well. Nevertheless, for receiving, efficiency of the antenna is secondary to signal to noise ratio. Don't worry about resonance, worry instead about getting away from the RF noise in your home.
 

ka3jjz

Wiki Admin Emeritus
Joined
Jul 22, 2002
Messages
22,866
Location
Bowie, Md.
This might be a possibility, too...


Mike
 

a29zuk

Member
Joined
Mar 6, 2005
Messages
725
Location
SE Michigan
The bottom line with HF reception is signal to noise ratio and when comparing 25ft of wire or 50ft or 100ft connected to a 9:1 transformer, in most cases the signal to noise ratio is about the same and the longer wire gives you more signal AND more noise but the SNR does not change. Going to a vertical might pick up distant signals better but the SNR could be worse due to higher noise. Yada yada, etc.
This is a good antenna handbook to read: www.monitoringtimes.com/antennabook.pdf

If you skip to page 3 to "Construction and Size", this kind of backs up what prcguy and ab3a are saying.

I always enjoy experimenting with antennas but I always keep the above in mind.

Jim
 
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