Dipole upside-down?

[JELAIR]

I'm a newbie at antenna-building

I have googled and searched this forum for answers, but didn't find anything conclusive.

Does a dipole have a 'right side up' ?

Or can either of the 2 pole-elements be the pole that hangs upwards/downwards?

I constructed a dipole for civil airband out of aluminum tubes, but didn't think about orientation when I soldered the connections.

This means I don't know which one of the 2 tubes (The dipole elements) goes to lead-in (On the radio-receiver's antenna-input) and which one goes to 'ground-in'.

I read that with monopole-verticals, you want transmitter and receiver to be the same orientation (Either both are upside-down, or both are rightside-up) to avoid phase-issues.
However, I am not sure if this is significant with dipoles (If a signal is received stronger or more faint, depending on the receiver dipole's orientation)

I attached some pictures of the tube-antenna I created.
It seems to work well, but now I'm wondering if a dipole can actually hang 'upside-down' and how that may affect reception (Weaker signals, stronger signals)
I can't really turn it upside-down to experiment, as the hooks it's hanging on only works one way (The way it's currently hanging on the horizontal black rod)

Thank you.
jacob.

 

[prcguy]

When modeling a dipole without any feedline attached it will have equal currents on both elements and it doesn't matter which end is up. When you attach a feedline in some cases current can flow on that feedline and it can radiate some. In this case there may be a slight difference in the radiation pattern swapping the element positions by 180deg.

If you put a choke balun at the feedpoint of your dipole it will reduce or eliminate feedline radiation and you can make one for the VHF air band by wrapping about 3 turns of your coax through a ferrite snap on core with a #43 mix and large enough to handle all the coax.

You might also shorten up the wires between your coax connector and the elements, they become part of the antenna length.
prcguy

 

[JELAIR]

When modeling a dipole without any feedline attached it will have equal currents on both elements and it doesn't matter which end is up. When you attach a feedline in some cases current can flow on that feedline and it can radiate some. In this case there may be a slight difference in the radiation pattern swapping the element positions by 180deg.

If I understand correctly; the dipole could therefore already be said to be either 'upside-right' or 'upside-down' no matter which poles I connected to 'lead' and 'ground' inside the antenna-plug.

So I take it I should not worry about the current antenna configuration (It is 'up-down neutral' in and of itself), but rather about the feed-line (Since the feed-line can 'spin' the antenna around if not handled correctly)

If you put a choke balun at the feedpoint of your dipole it will reduce or eliminate feedline radiation and you can make one for the VHF air band by wrapping about 3 turns of your coax through a ferrite snap on core with a #43 mix and large enough to handle all the coax.

I checked and no local stores have those, so I will have to order them.

When the feed-line is 'corrected', could I then face a potential problem of the dipole being 'upside-down'? (If the bottom-pole happens to be the one I soldered to lead-in)

Or will a 'corrected' feed-line make the antenna 'up-down neutral' (As it is without the feed-line)

You might also shorten up the wires between your coax connector and the elements, they become part of the antenna length.
prcguy

I had a bit of a battle with getting the wires to solder solidly to the aluminum, so that is a fight I will try to avoid for now (I can actually push and pull the aluminum tubes in length. They are air-condition tubes for cars that work like a pair of bellows, so they're somewhat flexible )

 

[popnokick]

JELAIR - Since you are into building antennas, there are many of us reading that would enjoy reading how what you made compares to this easy-to-make antenna (wire version... scroll down the page in the RR Antenna Wiki) -
Homebrewed Off-Center Fed Dipole - The RadioReference Wiki
Its "natural" resonance is in the civil air band.... although as you'll read in the description it covers MUCH more than aircraft comms.

 

[paulears]

You don't know which is which? Test meter?

As said above, it really doesn't matter.

What I just don't get is why you made such a dreadful looking and bodged up antenna - that looks from the photo to be wildly wrong in dimensions - as in around 60cm for each of the two elements. You can make them from any old bit of thin wire that would be pinball to the curtains, rather than need ropes and loads of tape to hold it together. Coax to a simple block connector, one Biot of wire up, one down. Your humongous diameter tubes might be the right length - difficult to scale from the photos, but that's got to be the most ugly antenna I've ever seen, and totally unnecessary.

 

[prcguy]

Since the OP is going for the VHF air band the wide elements will work in his favor to cover the roughly 20MHz of band width, assuming the dimensions are good.
prcguy

You don't know which is which? Test meter?

As said above, it really doesn't matter.

What I just don't get is why you made such a dreadful looking and bodged up antenna - that looks from the photo to be wildly wrong in dimensions - as in around 60cm for each of the two elements. You can make them from any old bit of thin wire that would be pinball to the curtains, rather than need ropes and loads of tape to hold it together. Coax to a simple block connector, one Biot of wire up, one down. Your humongous diameter tubes might be the right length - difficult to scale from the photos, but that's got to be the most ugly antenna I've ever seen, and totally unnecessary.

 

[JELAIR]

JELAIR - Since you are into building antennas, there are many of us reading that would enjoy reading how what you made compares to this easy-to-make antenna (wire version... scroll down the page in the RR Antenna Wiki) -
Homebrewed Off-Center Fed Dipole - The RadioReference Wiki
Its "natural" resonance is in the civil air band.... although as you'll read in the description it covers MUCH more than aircraft comms.

I have not made it beyond the very simple designs yet, so that one is still in the future

I'm just starting getting my feet wet with home-building. Previously I have only used pro-built antennas (That I didn't really question how was designed), so I'm pretty much still at square-one with all this (Learning a little bit more every day)

You don't know which is which? Test meter?

As said above, it really doesn't matter.

What I just don't get is why you made such a dreadful looking and bodged up antenna - that looks from the photo to be wildly wrong in dimensions - as in around 60cm for each of the two elements. You can make them from any old bit of thin wire that would be pinball to the curtains, rather than need ropes and loads of tape to hold it together. Coax to a simple block connector, one Biot of wire up, one down. Your humongous diameter tubes might be the right length - difficult to scale from the photos, but that's got to be the most ugly antenna I've ever seen, and totally unnecessary.

A test-meter... BIG embarrassing face-palm.

Yes, I will do that. Thank you
(Sometimes solutions are just so simple and straight-forward they become invisible )

Yes, each pole is about 60 cm long, and 6 cm in diameter.
The material is very thin and they're very light, even though they look heavy. They actually hang from a horizontal metal-rod that is standing a few feet in front of the curtain, so they don't hang in the curtain.

If you look at the pictures, you can see a very thin wire-dipole hanging just to the right side of the tube-dipole. I made that one before the tube-antenna. It's very thin copper-wire (About 1 or 2 mm thick), but the reception is about 3 dB lower than what I get with the tube-antenna.
With the wire-dipole I can not hear any of the control-centers clearly. But with the tube-dipole they come through ok. The signal appears to be around 3 dB higher with the tubes than the wires (I'm using SDR# and referring to the signal-spikes on the spectrum-display)

From pole-tip to pole-tip, the wire-dipole and tube-dipole are exactly the same length (Give or take a few millimeters), so the tubes themselves do something that's beneficial to the reception at my location (I don't know what it is, but perhaps the radio-waves just find the antenna more attractive than you do )

The looks are not important just yet, as I'm still early in my experiments with designs to learn what works and what doesn't work. So think of it as you would a temporary test-setup on your work-desk

 

[mule1075]

Your humongous diameter tubes might be the right length - difficult to scale from the photos, but that's got to be the most ugly antenna I've ever seen, and totally unnecessary.

Dude lighten up If it works it works and quite honestly you are being rude.

 

[majoco]

Is there a balun in that adapter that you have used - often they are used to go from 'rabbit's ears' type TV antennas to coax cable. That might be reducing your signal as the two dipole elements (in theory!) produce an impedance of 72ohms which you could connect directly to the coaxial cable without the adapter. The best solution is to take that adapter apart and modify the balun inside to be a ratio of 1:1 - you could use the wire that's already there - a couple of turns through the 'eyes' to the screw terminals and another couple to the coax cable connector. Then you have 'balanced' the antenna so it won't matter which way is up!

 

[jonwienke]

The biggest problem is using an unbalanced feedline on a balanced antenna. A balun is needed to fix that, or else the feedline is going to act as part of the antenna and cause all kinds of problems.

 

[JELAIR]

Dude lighten up If it works it works and quite honestly you are being rude.

Haha, thanks
I don't mind his opinion. The antenna is very much an experimental DIY antenna and 'looks' wasn't part of the equation when I built it
I do like the metallic tube-look though, but obviously the duct-tape and card-board backing is not exactly pretty

Is there a balun in that adapter that you have used - often they are used to go from 'rabbit's ears' type TV antennas to coax cable. That might be reducing your signal as the two dipole elements (in theory!) produce an impedance of 72ohms which you could connect directly to the coaxial cable without the adapter. The best solution is to take that adapter apart and modify the balun inside to be a ratio of 1:1 - you could use the wire that's already there - a couple of turns through the 'eyes' to the screw terminals and another couple to the coax cable connector. Then you have 'balanced' the antenna so it won't matter which way is up!

No balun inside. The connector I mounted on the antenna itself is 'empty' inside (Just bare metal and screws)
I don't know its exact impedance, but it's designed for 'normal' (In Scandinavia) TV/sat-box connections, so I would imagine it's aiming for 75 ohms (If it makes any real sense to talk about impedance of such connectors)
I attached a picture of one such plug opened up.

The white cable attached to the plug (The other end goes to the receiver's antenna input) is 75 ohms also.
It has what I think is a 'choke' molded into the cable (Not something I can open and modify) on each end. The product-description doesn't say anything specific about it, other than that it's a 75 ohm cable with ferrite beads (The ferrite isn't further specified)
So whether or not it acts as a balun I don't really know.

So all in all; antenna, plug and cable should be well-matched (In theory at least )

The 2 small wires going from the actual aluminum tubes to the the antenna-plug on the side, are speaker-cable.
It's not copper, but I don't know which kind of metal it is. It's from a cheap surround-sound setup which broke.
I don't know the impedance of those wires, but google says speaker-cable is generally low-ohm (typically single-digit ohms? I'm not sure)
I don't know how those 2 short wires exactly impact the antenna.

The 2 aluminum tubes themselves are, well, aluminum. That's all I know about them really
Very thin and coated with some type of resin on the outside (They are only conductive on the inside)
And formed in a zig-zag pattern (Like an accordion or a pair of bellows) which makes them stretchable and bendable (They're stiff enough to keep their shape when hanging inside, but not enough to withstand any kind of real wind pressure outside)
In the store where I bought them, they were in the car-parts section as some type of cooling-system or climate-control thing.

The biggest problem is using an unbalanced feedline on a balanced antenna. A balun is needed to fix that, or else the feedline is going to act as part of the antenna and cause all kinds of problems.

Thanks.
As said in the reply above; I don't know for sure what the effects of the ferrite-beads on the cable are.
There's a national FM-radio tower ~3½ km (~2.1 miles) from my location, so the out-of-band noise is already so high (No matter which cables or antennas I plug into the receiver) it's difficult to assess if I have extra noise from the cable.
I have been thinking about an FM-radio filter, but I'm not sure which ones are compatible with my goal of air-band reception (I've read some of them extend their attenuation somewhat into the air-band also, so I'm guessing that would be counter-productive)

 

[JELAIR]

Is there a balun in that adapter that you have used

I couldn't get the picture of the opened plug attached to the previous post, but it can be seen in a different post I made here: https://forums.radioreference.com/build-your-own-antenna/366589-where-do-antenna-actually-begin.html