Dual dipole theory question

Do you enjoy making your own equipment or antennas? Discuss construction and design in here.
Post Reply
beltane
Regular
Regular
Posts: 29
Joined: 09 Mar 2022, 11:03

Dual dipole theory question

Post by beltane »

Trying to be simplistic here so I can get my head round how things work. I have a dipole idea for a four element UHF/VHF dipole (quadpole?).

In simple terms I'm looking at the two halves of the dipole to be made of "V" shapes with one arm longer than the other. The long arm would be 1/4 wave 2m and the short arms would be 1/2 wave 70cm, with the feed points obviously being at the pointy bit where the two arms of each "V" meet. The whole thing would look a bit like a flattened out "X" with two shorter arms.

Ignoring feeder complications for the moment is there any reason why this wouldn't work better for dual band than a normal dipole? Surely if this were a better design somebody would be making and selling them, so I'm just wondering why I don't see any. Is my idea wrong?
User avatar
ch25
Top Poster
Top Poster
Posts: 1539
Joined: 03 Dec 2016, 11:07
Call Sign: Lemmy
Location: Poland

Re: Dual dipole theory question

Post by ch25 »

Is it similar to your idea?

https://www.eham.net/article/7005
WE ARE MOTÖRHEAD, AND WE PLAY ROCK N' ROLL
You can't have too many antennas...
Alan Pilot
Super Member
Super Member
Posts: 454
Joined: 21 Dec 2019, 07:24
Location: Anglesey

Re: Dual dipole theory question

Post by Alan Pilot »

I made one similar out of welding rod and it worked well.
LondonCbRadio
Regular
Regular
Posts: 94
Joined: 29 May 2018, 15:19

Re: Dual dipole theory question

Post by LondonCbRadio »

The reason why they don't sell them is due to the fact you will have trouble waterproofing that type of antenna. When it rains you would most likely end up getting a short due to the antenna being exposed to rain. As a loft antenna it's perfect.
paulears
Top Poster
Top Poster
Posts: 1025
Joined: 10 Jun 2007, 22:41
Call Sign: G4RMT
Location: North East Suffolk
Contact:

Re: Dual dipole theory question

Post by paulears »

If you look at things like log-periodic antennas they look totally different until you notice that they too are a number of different length dipole elements with their root going to a common point. All these designs use the fact that the impedance of an antenna is freuquency specific, and if they are the wrong length for the desired frequency, the impedance rises and they become a poor match and ineffective. The only thing to remember is that the gain at the wanted frequency is usually about the same or a bit less than just a dipole of the right length. In a dual bander, you have two or three that might be good matches at 144, 435 and a hiher or lower one - like maybe a VHF airband frequency. Hopefully, as one dipole gets less efficient, the next one comes up- which is a simplistic viewpoint of the log periodics I use for radio microphones - to get good coverage of one quite big range. 144 and 435 are too far apart to be much cop at say 240MHz - both dipoles are pretty unresonant, and just bits of metal in the sky. Like LondonCbRadio said - these things outside are just difficult to waterproof. There are commercial ones with proper solid waterproof parts and they're very expensive. That said - nowadays you could probably 3D design and print the parts to do it properly?
User avatar
ch25
Top Poster
Top Poster
Posts: 1539
Joined: 03 Dec 2016, 11:07
Call Sign: Lemmy
Location: Poland

Re: Dual dipole theory question

Post by ch25 »

paulears wrote: 12 Jan 2023, 20:13There are commercial ones with proper solid waterproof parts and they're very expensive.
I waterproofed antenna from link provided with hot glue. Lasted more than 10 years outside, I replaced it with different design. No sign of water in coax.

@OP
If you want dual band dipole build that one: https://www.eham.net/article/8808
WE ARE MOTÖRHEAD, AND WE PLAY ROCK N' ROLL
You can't have too many antennas...
beltane
Regular
Regular
Posts: 29
Joined: 09 Mar 2022, 11:03

Re: Dual dipole theory question

Post by beltane »

ch25 wrote: 12 Jan 2023, 00:02 Is it similar to your idea?

https://www.eham.net/article/7005
That's a similar sort of idea I think. Basically, my idea was to just make a two dimensional, sideways double "V" design with one shorter arm on each "V". It's to be mounted on a camera tripod to be portable so waterproofing isn't important. It would mostly be for things like setting up outside my tent while camping or putting on the roof of my car when trying to work satellites from a stationary spot with a handheld.

In my experiments with antennas so far they usually turn out drastically different than what they claim to be on paper. Seems like all the calculations and theory are just rough estimates of what any given design might do.

At the moment the best HT antenna I've used so far is a car thing with a woefully inadequate magnet that cost me three quid from China. I got it on a whim just because it was cheap and I thought it might make a camping antenna. I'd need to glue a much better magnet on it before I could ever use it on a car roof but indoors It actually works better than any of my other antennas costing many times more.
beltane
Regular
Regular
Posts: 29
Joined: 09 Mar 2022, 11:03

Re: Dual dipole theory question

Post by beltane »

@OP
If you want dual band dipole build that one: https://www.eham.net/article/8808
[/quote]

Actually that looks like a good one to try.
User avatar
ch25
Top Poster
Top Poster
Posts: 1539
Joined: 03 Dec 2016, 11:07
Call Sign: Lemmy
Location: Poland

Re: Dual dipole theory question

Post by ch25 »

This is mine, built recently.
4mm brass rods, plastic cutting board, fibreglass pipe.
sleeve.jpg
2m.jpg
70cm.jpg
You do not have the required permissions to view the files attached to this post.
WE ARE MOTÖRHEAD, AND WE PLAY ROCK N' ROLL
You can't have too many antennas...
User avatar
ch25
Top Poster
Top Poster
Posts: 1539
Joined: 03 Dec 2016, 11:07
Call Sign: Lemmy
Location: Poland

Re: Dual dipole theory question

Post by ch25 »

2mm.jpg
70ccm.jpg
You do not have the required permissions to view the files attached to this post.
WE ARE MOTÖRHEAD, AND WE PLAY ROCK N' ROLL
You can't have too many antennas...
beltane
Regular
Regular
Posts: 29
Joined: 09 Mar 2022, 11:03

Re: Dual dipole theory question

Post by beltane »

Looks like it's good for my camera tripod mount idea. I'll be using it on a handheld with about a meter or so feeder.
Post Reply