DB-224 wiring harness replacement

[n0wjh]

the local ham has a db 224 antenna we need a new harness ...has anyone ever rebuilt one ??
how would you rate the job from never again to easy..or easier cheaper to buy a replacement ???
did you use the 35 ohm cable it calls for ?
thanks terry N0WJH

 

[n5npo]

What exactly is wrong with the harness? Has it gotten water in it?
I have gotten several replacement harnesses FREE, but I have not done it lately.
I have some contact info that may still be valid.
73
Norm

 

[BirkenVogt]

I have had success replacing the harness on a single bay dipole, and have analyzed the way 222s and 224s are constructed.

Single loop impedance against mast = 100 ohm

To transform to 50 ohms you need a 1/4 wave segment of the intermediate impedance. So the 22x antennas first use a piece of RG-11.

Now you put two ends of RG-11 together in parallel, each one having a 50 ohm impedance and you get 25 ohms. So to transform it back to 50 ohms you need a 1/4 wave segment of 37.5 ohm cable, 35 ohms close enough, RG-83.

I am not sure if the 224 just uses one more tee and section of RG-83 but it should work.

I would not do it. Too many joints to leak and go bad over years of exposure. I would just find one to buy,

 

[prcguy]

The DB 224 elements are simply a 1/2 wave dipole with the hot side a 1/4 wave folded monopole and should have about the same impedance as a conventional 1/2 wave dipole. Same holds true for their 1/4 wave ground planes that have a 1/4 wave folded and grounded monopole for the vertical radiator. So, the impedance should be low but close to 50 ohms when against the the mast.

With that said, you can make a simple phasing harness from RG-59, RG-6 or RG-11 and I found using F type compression connectors and F type "T" adapters really simplifies the construction. Check out my VHF dipole array project and the phasing harness shown there will work on a DB-224. Scroll down on the link to find the pdf instructions.
http://forums.radioreference.com/build-your-own-antenna/109144-4-bay-vhf-dipole-array-project.html
prcguy

I have had success replacing the harness on a single bay dipole, and have analyzed the way 222s and 224s are constructed.

Single loop impedance against mast = 100 ohm

To transform to 50 ohms you need a 1/4 wave segment of the intermediate impedance. So the 22x antennas first use a piece of RG-11.

Now you put two ends of RG-11 together in parallel, each one having a 50 ohm impedance and you get 25 ohms. So to transform it back to 50 ohms you need a 1/4 wave segment of 37.5 ohm cable, 35 ohms close enough, RG-83.

I am not sure if the 224 just uses one more tee and section of RG-83 but it should work.

I would not do it. Too many joints to leak and go bad over years of exposure. I would just find one to buy,

 

[BirkenVogt]

The DB 224 elements are simply a 1/2 wave dipole with the hot side a 1/4 wave folded monopole and should have about the same impedance as a conventional 1/2 wave dipole. Same holds true for their 1/4 wave ground planes that have a 1/4 wave folded and grounded monopole for the vertical radiator. So, the impedance should be low but close to 50 ohms when against the the mast.

We have measured the impedance of a single loop close to a mast at around 100 ohms. The reason is that we cut the harness on a Celwave 1121 single loop and learned by experience that 50 ohm coax direct to the feedpoint does not work.

They use a 1/4 wave piece of RG-11 and you will see on the data sheet it is marked "transformer-do not cut". They meant it.

 

[prcguy]

This morning I tested a single dipole element from a DB Products DB408 UHF antenna and a VHF element from an RFS dipole array, which is an identical design.

With a short length of RG-142 50 ohm coax attached and the dipoles against a length of pipe, they both match beautifully across their stated bands with a 50 ohm match as measured on a Comet antenna analyzer. Out of band on the low end around 120MHz on the VHF version, the impedance goes down to 25 ohms and above it goes higher than 50 ohms. Measurement was very stable and predictable.

Without the pipe as a reflector the match still hovered around 50 ohms but there was a lot of hand interaction and the VSWR was about 2:1 at best. This repeats what I have measured in the past and my belief these dipoles are not 100 ohm but 50 ohm when mounted to a mast.

I also believe DB Products makes a more elaborate phasing harness than typically used by other companies and that a simple harness made from 1/4 wave or multiples of 75 ohm coax as described in my dipole array project will work just fine.
prcguy

We have measured the impedance of a single loop close to a mast at around 100 ohms. The reason is that we cut the harness on a Celwave 1121 single loop and learned by experience that 50 ohm coax direct to the feedpoint does not work.

They use a 1/4 wave piece of RG-11 and you will see on the data sheet it is marked "transformer-do not cut". They meant it.

 

[902]

Try this:

http://www.repeater-builder.com/db/pdfs/db-224-conversion.pdf

 

[BirkenVogt]

This one seems to be the reference they were trying to find:

www.repeater-builder.com/db/pdfs/db-224-af5u.pdf

The data sheet that mentions the 1/4 wave 75 ohm matching tx:

http://www.rfsworld.com/websearch/DataSheets/pdf/?q=1121-4

Here is a reference to the impedance of a single dipole (73 ohm) vs folded (292 ohm)

http://www.ece.mcmaster.ca/faculty/nikolova/antenna_dload/labs/Exercise1-5.pdf

All I have been able to find is the claim that bringing the dipole closer to the mast makes its impedance decrease but how much is in debate.

Prcguy, how did you connect said 50 ohm cable to your antennas? Unbolt the factory harness and bolt on your test 50 ohm cable? I will try to duplicate it as we have several antennas and masts to choose from.

Thanks
Birken

 

[prcguy]

I have spare DB408 elements and a new RFS antenna to play with and no harness attached. A short 50 ohm test cable with lugs was used.

The DB244 or 408 elements are not completely folded dipoles, which of course would have a higher impedance, maybe 200 ohm in free space. There is a ground side of the dipole completely shorted to the boom and the hot side is a folded monopole with one side grounded to the boom. This gives it more BW than a conventional 1/2 wave dipole but otherwise its about the same impedance as a conventional dipole.
prcguy

This one seems to be the reference they were trying to find:

www.repeater-builder.com/db/pdfs/db-224-af5u.pdf

The data sheet that mentions the 1/4 wave 75 ohm matching tx:

http://www.rfsworld.com/websearch/DataSheets/pdf/?q=1121-4

Here is a reference to the impedance of a single dipole (73 ohm) vs folded (292 ohm)

http://www.ece.mcmaster.ca/faculty/nikolova/antenna_dload/labs/Exercise1-5.pdf

All I have been able to find is the claim that bringing the dipole closer to the mast makes its impedance decrease but how much is in debate.

Prcguy, how did you connect said 50 ohm cable to your antennas? Unbolt the factory harness and bolt on your test 50 ohm cable? I will try to duplicate it as we have several antennas and masts to choose from.

Thanks
Birken

 

[talkpair]

It seems that in order to accurately measure the impedance of the element itself, you should be using a half-wave (multiple) section of cable cut for the mid point frequency, instead of some random length of cable.

Any length other than a half-wave multiple should be considered an impedance transformer the way I see it. By using a half-wave multiple, the unknown impedance of the element is brought back to the analyzer.

If the DB224 harness is made up of true odd-multiple quarter wave transformers, then the impedance of the element itself works out to 112.5 ohms on paper.
If I were to assume the elements are 50 ohms, then the impedance at the feed point connector is 112.5 ohms.

I have seen at least 2 different drawings of the harness, which both work in theory. The differences are mainly the types of cable used and number of junctions involved.

It might be worth mentioning that selection of cables with a velocity factor of .66 is important if you want the cables to reach the elements and not end up with slack to deal with on the mast.

 

[prcguy]

You are correct about the 1/2 wavelength cables for repeating the impedance at the analyzer, however I've measured these specific elements several times over the years with various length cables and they have all come out around 50 ohms.

If the feedpoint of an antenna is 50 ohms and its fed with 50 ohm coax then feedline length will not matter. The elements I tested had a nearly perfect match across their stated range and the measurement was very stable. If there is a feedline to antenna mismatch the measurement can sometimes be unstable when running your hand up and down the feedline due to common mode currents and this was not the case here.

The same loop design that is grounded in the center (making it around 70 ohms free space in my opinion) is used by many other mfrs and some of them use simple multiples of 1/4 wave in 75 ohm coax for the phasing harness as shown in my dipole array project. There is nothing secret that DB Products did to make their elements different than the other mfrs.

The simple phasing harness or power divider made of 1/4 wave or multiples is basically a Wilkinson power divider but without the 100 ohm balancing resistors across the outputs, which is not ideal. I suspect DB Products has simply made a better, although more complicated power divider compared to other companies.
prcguy

It seems that in order to accurately measure the impedance of the element itself, you should be using a half-wave (multiple) section of cable cut for the mid point frequency, instead of some random length of cable.

Any length other than a half-wave multiple should be considered an impedance transformer the way I see it. By using a half-wave multiple, the unknown impedance of the element is brought back to the analyzer.

If the DB224 harness is made up of true odd-multiple quarter wave transformers, then the impedance of the element itself works out to 112.5 ohms on paper.
If I were to assume the elements are 50 ohms, then the impedance at the feed point connector is 112.5 ohms.

I have seen at least 2 different drawings of the harness, which both work in theory. The differences are mainly the types of cable used and number of junctions involved.

It might be worth mentioning that selection of cables with a velocity factor of .66 is important if you want the cables to reach the elements and not end up with slack to deal with on the mast.