• Effective immediately we will be deleting, without notice, any negative threads or posts that deal with the use of encryption and streaming of scanner audio.

    We've noticed a huge increase in rants and negative posts that revolve around agencies going to encryption due to the broadcasting of scanner audio on the internet. It's now worn out and continues to be the same recycled rants. These rants hijack the threads and derail the conversation. They no longer have a place anywhere on this forum other than in the designated threads in the Rants forum in the Tavern.

    If you violate these guidelines your post will be deleted without notice and an infraction will be issued. We are not against discussion of this issue. You just need to do it in the right place. For example:
    https://forums.radioreference.com/rants/224104-official-thread-live-audio-feeds-scanners-wait-encryption.html

Dual Band Dipole

Status
Not open for further replies.

KD2DXF

Member
Joined
Mar 8, 2010
Messages
424
Location
Utica, NY
After seeing this Project I decided to give it a try. Spent a whole $13.38 at Lowes for the supplies. My variation involved using metric washers, so I only needed one pair to do the job. supplies and pics below. My next project is the Dual Band J-Pole The J-Pole will be more challenging. My plans are to get new coax, properly tune (when i find someone local who would lend me a tuner), weatherproof, and mount outside somewheres. Nice part about this, is you can fold and go if needed for portable emergency use.

Supplies bought:
1 18 x 2 1/2 x 1/4 piece of plexiglass
4 #6/32 1" long machine bolts
3 #6/32 Wing Nuts
2 Metric # M5-155
2 1/8ths thick brass rods 3 foot lengths. (didnt have stainless steel)
 

Attachments

prcguy

Member
Joined
Jun 30, 2006
Messages
7,827
Location
So Cal - Richardson, TX - Tewksbury, MA
I'll comment here even though its not positive. The original designer of this antenna must not have realized a 1/2 wave dipole is also resonant on odd harmonics with a low impedance. This means a VHF 150MHz dipole will also resonate at 450MHz with a low impedance at the feedpoint.

If you add a resonat 450MHz dipole to a 150MHz dipole the feedpoint impedance on UHF will be half of what you want and similar if not equal currents will flow in both dipoles. In other words its not a good idea.

On the other hand an end fed 1/2 wave like a J-pole will also resonate on even harmonics like one cut for 150MHz will be potentially useful at 300MHz and so on but with a compromised pattern on the higher freqs. The J-pole is a better candidate for a VHF/UHF dual band design since the VHF and UHF sections should operate more independantly and not load down the feedpoint from two sets of elements fighting for attention on the same band.
prcguy
 

KD2DXF

Member
Joined
Mar 8, 2010
Messages
424
Location
Utica, NY
I'll comment here even though its not positive. The original designer of this antenna must not have realized a 1/2 wave dipole is also resonant on odd harmonics with a low impedance. This means a VHF 150MHz dipole will also resonate at 450MHz with a low impedance at the feedpoint.

If you add a resonat 450MHz dipole to a 150MHz dipole the feedpoint impedance on UHF will be half of what you want and similar if not equal currents will flow in both dipoles. In other words its not a good idea.

On the other hand an end fed 1/2 wave like a J-pole will also resonate on even harmonics like one cut for 150MHz will be potentially useful at 300MHz and so on but with a compromised pattern on the higher freqs. The J-pole is a better candidate for a VHF/UHF dual band design since the VHF and UHF sections should operate more independantly and not load down the feedpoint from two sets of elements fighting for attention on the same band.
prcguy
Hey im open to any comments and suggestions. I'm not as well versed in antenna theory as id like to be. What I most likely will do to this is make a simple 2 meter dipole. since I do not use UHF a whole lot.I had it hooked up to my dual band chinese radio, and was making decent range with distant repeaters on VHF, not so much UHF. Another option could be to have VHF on one side of the plexiglass, and UHF on the other side, and have 2 different coax going to their respective radio.
 

popnokick

Member
Premium Subscriber
Joined
Mar 21, 2004
Messages
1,885
Location
Northeast PA
GREAT that you're experimenting with antennas! That's part of what ham radio is all about. But your next priority should be to get something that will permit you to make some measurements... even if rudimentary... of the effectiveness of your antennas. You don't have to spend a lot to get a simple combo RF Field Strength Meter/ SWR Meter. Amazon has one made by Opek for $30. You can spend a LOT more for high end meters.... and don't be confused by all the ghost-hunting EMF meters out there. You want an RF Field Strength meter that specifies what range it covers (e.g. 1.7 - 150 mHz). MFJ has them as well. And you may be able to find a used one somewhere. By spending a few dollars you'll be able to compare one antenna to another (e.g. the dipole to the J-pole) by comparing field strength of each antenna, SWR, etc. Enough so that you can say one was better than the other.
 

prcguy

Member
Joined
Jun 30, 2006
Messages
7,827
Location
So Cal - Richardson, TX - Tewksbury, MA
The UHF element should be mostly invisible and out of the circuit on 2m.
prcguy

Hey im open to any comments and suggestions. I'm not as well versed in antenna theory as id like to be. What I most likely will do to this is make a simple 2 meter dipole. since I do not use UHF a whole lot.I had it hooked up to my dual band chinese radio, and was making decent range with distant repeaters on VHF, not so much UHF. Another option could be to have VHF on one side of the plexiglass, and UHF on the other side, and have 2 different coax going to their respective radio.
 

wbswetnam

Member
Premium Subscriber
Joined
Oct 11, 2005
Messages
1,569
Location
DMR-istan
I also made this antenna. I got it from KG0ZZ Dave Tadlock's Youtube channel:http://youtu.be/RcO5WAiksNI
I followed Dave's instructions to the letter but I still had trouble with getting a good SWR reading on both bands. I suspected that the two parallel sets of elements were interfering with each other. I got it to work much better by reversing the bottom element (remove, flip it around 180 degrees then reattach), then bending the elements apart on the top and bottom by 20 degrees.
 
Last edited:

wbswetnam

Member
Premium Subscriber
Joined
Oct 11, 2005
Messages
1,569
Location
DMR-istan
I took a photo of the 2m / 70cm vertical dipole today, showing how the elements are spread apart from each other.
 
Last edited:

SCPD

Member
Joined
Feb 24, 2001
Messages
65,126
Location
Virginia
Play

It's great you're experimenting.

Many people thru the years have used multi-element antenna such as these on HF.
They take rotor cable, cut it into dipoles, and join the ends together just like you did, and work the world. (15 and 40 can be worked on the same dipole).

Remember the best way to tell if an antenna works, is to throw it up and see how it works.
Here's one plan I found for an hf multi band made out of ribbon cable.

6 Band Wire Antenna
 
Status
Not open for further replies.
Top