Home made antenna with RG6/U Coax

ChrisJefferies

Hi folks,

I've just bought a Uniden UBC72XLT scanner and I want to put a home made antenna up in the loft as I'm not getting much on it at the moment.

I've found a few links on the net, http://www.hamuniverse.com/scannerantenna.html looks to be about as basic as you can get so I'm going to have a go at making it.

I remembered that I have a brand new 100m roll of RG6/U in my garage that I've never used. After some searching on RG6/U I think it's 75 ohm where as my manual for my scanner says it's antenna is 50 ohm.

I'm concerned that if I use this 75 Ohm coax instead of (RG58 I think if that's 50 Ohm) I'll damage something in my new scanner.

Can someone tell me if this is true or is the 25 Ohm difference not going to be a problem.

Thanks

Chris

Taildragon

Hi Chris

Go ahead and build it, you'll be fine. It's only a simple 1/4 wave groundplane ("Marconi") antenna though, so don't expect too much from it, especially if it's installed in a loft. It's a good starting point, antenna experiments are cheap and very educational.

75 vs. 50 Ohm is irrelevant for almost all receiver setups, and most transmitter setups. I assume that your connector on the scanner is BNC? The centre pin of a 75 Ohm BNC plug is slightly bigger than a 50 Ohm one, so I'd use a 50 Ohm unit, otherwise you might (possibly) have problems later on as the female portion of your socket may "spread" and cause intermittent contact.

Let us know how you get on!

Taildragon

Huge repository of useful links for antenna experimenters here:

http://www.ac6v.com/antprojects.htm

ChrisJefferies

Great, thanks for clarifying that and telling me about the bnc plug size difference.

I had visions of my scanner melting before my eyes :eek:

Just my luck I have 100 meters of perfectly good wrong cable!

Oh well, I've learnt a few things from having it already so it 's not all bad.

Thanks again.

Regards,

Chris

ChrisJefferies

Just want to say thanks to everyone who's given advice these past few days.

Tried my antenna last night but it didn't work. Took it down and tested it with a connectivity meter and found that a connector that had been in the cable when I found it was duff so I replaced it, put it back up again and it worked

I'm in the midlands of Ireland and managed to pick up Malin Head coast guard, EI2PAR Phoenix Amateur Radio Club, Blanchardstown, someone in Tyne and Wear UK and Dorset UK. Oh and loads of aircraft and Dublin Control Tower.

I'm still buzzing this morning I didn't realise the excitement you feel when you pick up a transmission and realise it's come from miles away. I'm still not sure I believe my small antenna can pick up signals as far as the UK.

Obviously radio operators keep a log of their contacts made, do scanner operators do similiar?

If so do you just note it down in a diary, buy a special log book, download sheet's off the net or use logging software on PC?

Thanks again.

watty

Any of those that suits you.

Squared paper ringback notebook
Spreadsheet
MixW digital Modes SW.

Occasionally I've had written postcard, or email report from a Listener. A couple of reports from England with me transmitting in Limerick on VHF.

I think one mainland "SWL" HF report.

Google SWL QSL Card

ChrisJefferies

Thanks Watty. I think I might jump in at the deep end and log things with the Ham Radio Deluxe software, dreaming of the day when I can it use it to rotate my antenna and use it with a Transceiver.

ChrisJefferies

Hi folks,

Next question

What are the preferred connectors for connecting the antenna to coax.

I have a male BNC connector at the scanner end which is fine.

The other end I obviously need to connect it to the antenna. I was going to put a female BNC connector on the short coax tail of antenna and have a male connector at the end of my coax (feed line I think it's called). A quick search around ebay and it seems that female BNC connectors aren't that common so I'm wondering whether I'm thinking along the right lines.

Are BNC's only really used at the equipment end and other types of connectors are used to join pieces of Coax together?

Thanks

Taildragon

Hi Chris

There are 3 common type of connector for co-ax

BNC
PL259/SO239
'N' type

of these, the PL259 (and its mating SO239) connector is probably the most common. BNC connectors tend to be used more at VHF and UHF because they offer a better impedance match to 50 and 75 Ohm cables.

Don't get too hung up on impedance matching with your setup, unless you get a wild mismatch you'll never notice the difference. I'd happily use standard 75 Ohm co-ax, (cheap) TV co-ax connectors, or 'F' type satellite receiver connectors in an installation such as you've described - they only cost a few cent each.

As for terminating your loft-mounted antenna, I'd just solder the feeder directly to it, or make a good mechanical connection using "choc-block" or similar.

In-line female BNC connectors are available, most commonly "crimp on" types which need a special crimping tool. Solder type connectors are available, but are rare. I normally use a female/female barrel connector to join two male BNC.

'N' type connectors have almost identical characteristics to BNC, but can handle larger diameter cables. They are rarely seen on amateur equipment - more common in lab environments and on test equipment.

ChrisJefferies

Thanks TailDragon.

That's good to hear, so I've got all I need to tinker with for now and no excuses to have another go at my next antenna.

Thanks again

ChrisJefferies

Here's my next one...

I have my original quarterwave groundplane antenna in the loft and it's picking up the Dundalk 2M repeater ok. They were doing a CW class last night on a different frequency but I couldn't pick them up unfortunately. So as another experiment I've made a quick 2M Dipole.

My question is, am I better off replacing one antenna with the other or is having two of them in the loft better, connecting them together and then feeding them to one coax down to my scanner?

Taildragon

Hi Chris

Running two antennae in parallel (as you've described) isn't best practice, but you can't hurt anything by trying it. Who knows? You might make a happy discovery!

I can think of three common scenarios for running antennae in parallel: bandwidth, frequency diversity, and antenna gain.

In the first, we'd run dipoles of slightly different lengths in parallel to achieve a broadband antenna (normally on HF); in the second, we'd want to feed two antennae on two (or more) very different frequencies using a single feeder; in the third, we'd build a "phasing harness" to run high-gain Yagi antennae in parallel.

Addressing your inability to hear the CW net, there are a number of possible reasons for this.

- Repeaters are on high sites, you may have been trying to listen to low lying stations

- 2M repeaters normally run FM, which is vertically polarised by convention. 2M SSB/CW is usually horizontally polarised. Your antenna is vertically polarised.

- Your scanner may not be able to detect the signals at all. If they were using true CW (A1A) your receiver may not be suitable.

Do you have the option of putting an antenna on your roof or chimney? I think you'll get much more out of your receiver if you can improve your antenna location. You don't need to spend much (or any) money to achieve this, a simple chimney lashing kit and a 3M pole would give you a very good mount for VHF/UHF antenna experiments. Don't waste your money buying antennae - make them!

ChrisJefferies

Ah ok.

I do have the option to put an antenna on my chimney. I was thinking of building a 2M Slim Jim to put up there.

I also have the option to put something up on my garage. I'd need to run maybe 100-150ft to do it. Would there be any problems having a cable that long or would it be best just to stick to the house for such a small powered scanner?

If 2M can be vertical or horizontal, do people have two antenna's orientated both ways to receive both planes?

I believe the Dundalk club where just transmitting on "145.400 FM". When I see FM, should that tell me that it's vertically polarised or have I understood it wrong and FM can be on both planes?


Am I driving everyone nuts with all these questions yet :rolleyes:

Taildragon

Hi Chris

Well, I can't speak for anybody else who might be reading this thread, but you haven't driven me mad (yet). Some would say that I got there a long time ago ;-)

Polarisation.

Most FM activity (whether amateur radio, PMR, emergency services, broadcast FM etc.) is vertically polarised. The reason is obvious if you think about it - it's optimised for portable/vehicle use where vertical antennae (whips) are more convenient. There are propogation differences too, but that's not pertinent to your question.

By convention, amateur radio SSB & CW activity is horizontally polarised. A typical 2M FM/SSB/CW arrangement would be a horizontally-mounted Yagi on a rotator, with a vertically polarised omni antenna mounted above it on the same mast. Two runs of feeder down to the radio, and a changeover switch at the bottom.

Antenna

A "Slim Jim" is a good choice of antenna at this stage. You can build it for a few Euro, and it gives a moderate amount of gain while maintaining the omnidirectional properties you need (for now).

About the long co-ax run.

Now here I'm going to swim against the tide a little. How many times have you been told that so-and-so coax is useless and too lossy? I'll bet that there's no shortage of people ready to tell you that a 100 to 150 run of RG6 is a waste of time. OK, there are better(?) less lossy cables out there, but let's look at the real world for a moment.

Are you familiar with the 'S' meter? Most serious receivers have one, even if it is little more than an ornament. If you can hear a station at S7, and can understand all that's being said perfectly well, you'd be happy, yes? If you can hear that same station at S6, equally clearly, would you be any less happy? No?

OK, lets put some numbers on this. A 150ft run of RG6 has a loss of about 5dB at 145.500 MHz. In other words, about 50% of your received signal is wasted in cable losses. That sounds bad, doesn't it? What if I tell you that your 50% loss is less than 1 single 'S' point?

Now, there's an argument that in weak-signal work, every fraction of a dB counts. I won't argue with that, except to say that mass-produced consumer grade receivers and scanners are not anywhere near that league.

Getting your Slim Jim up high in the clear will gain you far more than you stand to lose in feeder losses.

The bottom line? If you spend your money on "high performance, low loss, gold plated, shiny black, must-be-good-because-it's-dear" coax then you're a sucker. Ordinary CT100 (satellite) coax is perfectly good for most installations and costs much less than €1 per metre. If you want really low loss feeder, for a really good price, then open-wire feeder is the way to go, and you can make it yourself!

ChrisJefferies

Thanks for the reply TailDragon, it's most appreciated.

With all that you've told me I should be able to build a few more antenna's now and do some more experimenting.

This is the first time I've heard of an S meter. I'll do some reading up on them.

Thanks.

watty

Cable loss below 1GHz is attenuating cosmic & Atmospheric noise as well as signal.

It's the SNR that counts. At HF cable loss on receive is irrelevent.

At VHF it not so important on receive. A bit more important at UHF.

But on Transmit it's important. So I have PA for VHF SSB in attic and drive with 3 to 5W from ground level.

Open wire feeder hardly used above HF as the dimensions must be more accurate. The big advantage is power. RG58 is not going to be happy with 10kW.

Disadvantage of open wire feeder is it can't be near anything and blocking common mode current more problematic. (Coax at low frequency wind coax on a ferrite, like old LOPT core. At high frequency a two turn loop of coax about 10cm diameter is common mode choke).

S= Signal.
S-meter
They are not accurate and vary considerably between radios.
S9 is traditionally perfect signal
S1 or S2 poor to unusable.
Originally it didn't relate to dB, i.e. you couldn't have more than 9 as that meant perfect. but it has come to mean a log (dB) measure of signal so you now hear very strong signals described as 20 over 9 (S9 + 20dB)

The International Amateur Radio Union (IARU) Region 1 agreed on a technical recommendation for S Meter calibration for HF and VHF/UHF transceivers in 1981.[1]

IARU Region 1 Technical Recommendation R.1 defines S9 for the HF bands to be a receiver input power of -73 dBm. This is a level of 50 microvolts at the receiver's antenna input assuming the input impedance of the receiver is 50 ohms.

For VHF bands the recommendation defines S9 to be a receiver input power of -93 dBm. This is the equivalent of 5 microvolts in 50 ohms.

The recommendation defines a difference of one S-unit corresponds to a difference of 6 decibels (dB), equivalent to a voltage ratio of two, or power ratio of four.

Signals stronger than S9 are given with an additional dB rating, thus "S9 + 20dB", or, verbally, "20 decibels over S9".

via Wikipediia S-Meter

The article correctly suggests that most S-Meters are in fact just measuring the voltage of the AGC signal. Professional designs may have an RSSI path independent of AGC that can output 0 to 3V for perhaps 90dB signal range in manner that can use a linear scale for dBuV signal.

The origin of 1 to 9 for S meter is the R S T code
http://en.wikipedia.org/wiki/RST_code
A method of logging or giving reports of radio reception quality without a meter or any test gear at all, originally.

Taildragon

This thread was started by Chris, who (by his own declaration) is a total beginner with a budget hand-held scanner, a length of coax and (most importantly) an eagerness to experiment and learn.

Its not about

- Transmitters, 10kW or otherwise
- Common mode currents
- Current baluns
- Watty letting everybody know how clever he is

Please don't kill this discussion stone dead with "techno-babble", I implore you. It would be much more helpful if you were to explain relevant aspects of the topic in language that a beginner can understand.

Liameter

May I add that another common connector type (probably THE most common) is the "F" plug. It is specifically used with 75 Ohm cable (such as RG6, WC100, WF100) for UHF TV and for satellite TV. However, if you are already using 75 Ohm cable, you might as well use the "F" plug, which is pretty simple to fit.
http://www.google.com/search?&q=How+to+fit+'F'+Connectors

ChrisJefferies

Ha ha, yes that was abit technical for my novice skills Watty but all replies are greatfullly received You'll have to dumb it down a little for me I'm afraid

I'm learning so much from you all and haven't had so much fun tinkering for ages (if ever).

I'm going to build my Slim Jim tonight and if the weathers dry, brave going up on the roof to fix it to my chimney. Or I might spend some time with the wife, I'll see how my chances are looking...

Oh and the half wave dipole vertically polarised (I'm learning) didn't seem very good (98 cms total length for 2 Meters, is that right???) So I connected up the quarter wave ground plane antenna next to it and I think I can get a better signal than just one or the other. Probably my imagination though without having anything to measure it with.

Have a good weekend all...

Taildragon

98 CM (980MM) sounds like a reasonable number all right. There's a useful frequency-to-wavelength calculator here:

http://www.onlineconversion.com/frequency_wavelength.htm

This will give the wavelength in "free space" - it will only be correct for a theoretical antenna. In practice, your antenna will be resonant at a shorter dimension than the theoretical 1/2 wave length; this is caused by a phenomenon known as "velocity factor".

If you place two or more resonant antenna elements in close proximity then they will affect one another, even though they are not touching and appear to have no physical connection. That effect can be used to our advantage when we distort the antanna pattern to achieve "gain" in a particular direction.

One example of this is the Yagi antenna - a TV antenna for terrestrial reception is a very common example of a Yagi.

Although we speak of antenna "gain", there isn't any real gain as such. We are just focusing the antenna pattern in one direction (or plane) at the expense of others. You could think of a Yagi as the radio equivalent of an optical telescope.

http://www.yagiantenna.net/

ChrisJefferies

Thanks Liameter for the F plug info. I took down an old Chorus TV dish yesterday and found a few of those type of connectors in the cable. Very sturdy little things.

I managed to put my Slim Jim up on the TV antenna pole mounted to my chimney. For all of you hams that regulary climb up high things I salute you! I only live in a bungalow but I wouldn't want to be climbing up there too often :eek:

It paid off though because now I can pick up the Kippure, Co. Dublin IO63UE repeater along side the Dundalk one. So I've gone from hearing nothing to picking up 2 repeaters 40-60 miles away so I'm really pleased. Again, thank you for all your help.

I have three more questions :rolleyes:

At the minute my Slim Jim is bare copper. What would happen if I painted or laquered it, would it stop its ability to receive?

Does it matter what diameter of wire/tube an antenna is made of? For example, which of two antennae of the same design, one made of thick copper pipe and one made of single core copper wire would be better, swr being the same? Does the thicker pipe have more surface area and therefore soak up more rf.

What's the typical way to water proof connections in the feed line? Do you smother the connectors with a hot glue gun, silicon, heatshrink or just leave them exposed?

Thanks for the yagi info Taildragon. As all my traffic comes from the east I think I'll build one of them next and point it that way to see what happens. I'm not going up on the chimney again anytime soon though...

Liameter

The velocity factor for coaxial cable is around 66%. In other words, the length of coax will be 60% of the free-space wavelength.
See http://www.nr6ca.org/vf.html

Paint and lacquer has no discernible effect on an antenna.

You can increase the bandwidth of an antenna by using thicker wire (or tube instead of wire).

I use silicone (not "silicon") grease to waterproof connections.
http://www.satcure.com/tech/silicone.htm

DON'T confuse this with "silicone rubber", which absorbs moisture!

You can also waterproof them with hot-melt adhesive, self-amalgamating tape, Denso tape, or put heatshrink sleeve over silicone grease to hold the grease in.

ChrisJefferies

Thanks for the reply Liameter, I'm sure Taildragon will be glad of the rest.

That silicone grease sounds like great stuff. When I come up with an antenna that's going to be more permanent that looks lie the stuff to use.

Well that's it, I've finally run out of questions for now

Thanks everyone, you've been very helpful and welcoming to the hobby of ham radio...

Taildragon

ChrisJefferies wrote: »

...As all my traffic comes from the east I think I'll build one of them next and point it that way to see what happens. I'm not going up on the chimney again anytime soon though...

I know what you mean about climbing on the roof - not everybody is comfortable with that kind of situation. I'm happy enough as long as it's safe and I'm wearing my climbing harness securely "clipped on". Some lunatics like to do "free climbing" - have a look at this YouTube video - I got a sick feeling in the pit of my stomach just watching it. I suppose you'd be just as dead from a 50ft. fall as a 2000ft. one though.....

http://www.youtube.com/watch?v=txdv_oNq81I

When you get around to building a Yagi, here's an excellent calculator for you:

http://www.k7mem.150m.com/Electronic_Notebook/antennas/yagi_vhf.html

I used that to design a 6-element Yagi recently, using some stainless steel welding rods and a length of aluminium "C" channel rescued from a skip.

Ther are some other very useful JavaScript utilities in the sidebar,

Mike 1972

Ive used to play around with homemade aerials. (Still have copper Jpole back home but havent used it in over ten years now)

A couple of things ive always wondered about Jpoles
Whats the difference between a Jpole and a Slim Jim
What factors determine the bandwitth of a Jpole
What effect does varying the distance bwtween the conductors in the matching section have
What are the advantages and disadvantages of using a 5/8 wave radiating section (as opposed to halfwave)
Whats the best type of balun to use
If one wanted to use build a Jpole in a confined space (e.g. an Attic) would an Lpole (with the matching setion perpendicular to the radiating section) work just as well

Is there any reason why coaxial dipoles dont seem to be widely used ?

ChrisJefferies wrote: »

At the minute my Slim Jim is bare copper. What would happen if I painted or laquered it, would it stop its ability to receive?.

No

ChrisJefferies wrote: »

Does it matter what diameter of wire/tube an antenna is made of? For example, which of two antennae of the same design, one made of thick copper pipe and one made of single core copper wire would be better, swr being the same? Does the thicker pipe have more surface area and therefore soak up more rf.

Thicker conductors (even if hollow) = more bandwidth (up to a point anyway) impedance drops slightly though.

Is copper not kinda expensive these days for making aerials out of ????

Taildragon

A couple of things ive always wondered about Jpoles
Whats the difference between a Jpole and a Slim Jim

None. Same animal, different name. It's an end-fed Zepp, in fact.

What factors determine the bandwitth of a Jpole

Assuming it's resonant, then the surface area of the radiating element is the dominant factor. "Fatter" elements are less lossy too if they are made from less-than-optimum materials. Silver-plated copper or brass good, galvanised steel bad.

What effect does varying the distance bwtween the conductors in the matching section have

It will vary the matching impedance. It's a 1/4 wave stub.

What are the advantages and disadvantages of using a 5/8 wave radiating section (as opposed to halfwave)

None that I can see.

Whats the best type of balun to use

The best balun is no balun.

If one wanted to use build a Jpole in a confined space (e.g. an Attic) would an Lpole (with the matching setion perpendicular to the radiating section) work just as well

Probably. Matching to the feeder may not be quite as simple as with the J-pole, though. What's the advantage?

Is there any reason why coaxial dipoles dont seem to be widely used ?

If you mean dipoles made from co-ax (bazooka), it's because they're lossy, heavy, and relatively expensive to make. If you mean a co-axial (sleeve) dipole, then I'd say that for the size, cost, and weight there are more attactive options such as the co-linear, or even a folded dipole.

Mike 1972

Taildragon wrote: »

The best balun is no balun.

But if one is using a coaxial (unbalanced) feeder with an inherently balanced aerial one should have a balun of some sort right ?

Taildragon wrote: »

Probably. Matching to the feeder may not be quite as simple as with the J-pole, though. What's the advantage?

At some frequencies a conventional J-pole (three quarters wavelength) might be a bit large going into a confined space. For use in an attic an L pole might be more advantageous

Taildragon wrote: »

If you mean dipoles made from co-ax (bazooka), it's because they're lossy, heavy, and relatively expensive to make. If you mean a co-axial (sleeve) dipole, then I'd say that for the size, cost, and weight there are more attactive options such as the co-linear, or even a folded dipole.

Yes but the coaxial (sleve) dipole is wouldnt need a balun and could even be used as part of a yagi ? Impedance matching with a folded dipole can be problematic.

Whats the difference between the colinear and stacked dipoles or are they different names for the same thing ?

watty

Stacked dipoles can implement a colinear. (suitably spaced and combined array)

A colinear can be also made from inline spaced monopoles on a single feed with either capacitive or inductive sections to space and connect the stack to the single feed point. (Watson 300)

A colinear can be made from an array of patch antennas or slot antennas on a single feed, or using printed wilkinson combiners.

Taildragon

But if one is using a coaxial (unbalanced) feeder with an inherently balanced aerial one should have a balun of some sort right ?

Not necessarily so. Using a truly balanced antenna with an unbalanced feeder isn't in itself a problem. Poorly designed or sloppily installed antennae may not, in fact, be balanced and that can give rise to several problems (the common mode currents that Watty alluded to earlier in this thread amongst them). Should such a situation arise, a common solution is to wind a few turns of the co-ax feeder into a coil, close to the feedpoint of the antenna. This is not a true balun, it is an RF choke. It treats the symptoms rather than addressing the cause, if you will.

Well engineered baluns are lossy. Badly designed or sloppily installed baluns may be very lossy.

The J-pole antenna (at VHF or UHF) is relatively easy to build accurately, and easy to adjust for good match and resonance, obviating the requirement for a balun in most cases.

At some frequencies a conventional J-pole (three quarters wavelength) might be a bit large going into a confined space. For use in an attic an L pole might be more advantageous

Ah, I think I see what you mean, now. Are you suggesting that the radiating element be constructed perpendicular to the matching section? If so, that would be a “classic” Zepp antenna configuration. It see no reason why that would not work as well as a J-pole, observing the usual caveats regarding clearances from pipework and electrical wiring in the loft.

watty

Most cheap TV aerials are technically balanced aerials (indoor or Yagi) and don't have a balun. The better ones do.

Dude111

Taildragon wrote:

antenna experiments are cheap and very educational.

Indeed.....

I have heard some pretty interesting things used as antennas which work quite well!!