DB-304 Wiring Harness

[nukesnooper]

I'm aware of the method of lengthening the elements, but the harness is still the biggest enigma. I'm just not sure how I would go about fabricating a reliable phasing harness that would work properly with the array. I'm just not that savvy.

Thanks again to everyone for your suggestions.

 

[talkpair]

Here's a link to the modification prcguy is talking about:

Modify a Decibel DB-224 for the 2-Meter Ham Band

Because band split of your DB304 is higher than the one on that web page, the screws used would need to be longer.

 

[nukesnooper]

OK. I'm still interested in the element mod. Thanks for the link! I'd seen several articles about that in the past, but my curiosity lies in the spacing between the tips of the elements. Stainless steel screws / bolts would be no problem to source. My hunch is that there needs to be a certain distance between the tops and bottoms of the elements that is carefully calculated. As I mentioned earlier; the space between the top to the bottom of the two bottom and two top elements is 28". The melon scratcher is; the distance between the top of the bottom 4 is only 24" from the bottom of the top four. I would have thought that all 4 elements would have been equally spaced. Any ideas on this oddity? What I'd like to do is to rotate the lowers at a 90 degree angle from the top, giving me a more Omni-directional radiation pattern.

In fact; I'll take it one better: "Suppose" I were able to find some aluminum tubing with the same diameter (inside and out), cut the ends off of the elements, insert some smaller copper tubing that would insert inside of the elements to stabilize them. Would the presence of the inner tubing inside the elements create another issue? Best case scenario: Find a couple of DB224's and cut the element ends off, leaving enough length to only have one weld joint at each attachment? OK, enough brain storming (or brain farting).

Then; I'm still left with trying to match a phasing harness. With the exception of dulling of the outer jacket of the phasing harness; it looks to be intact. Obviously; that's not the case. My hunch is that it took a lightning hit at some point in its life and the "opens" are inside the "Y" connections, which are covered by the thick black plastic covers. If I took a Dremel (or similar) tool and cut the plastic shields off of the junctions and only found that the solder joints were smoked; I could re-solder the connections - but at what lengths to match the antenna frequency? At the higher frequency, my guess is that they'd be too short to work with (enter wire stretcher) Worst case scenario: The coax is completely hosed and unusable. Back to square one. I'd have to build a harness from scratch. I'm not opposed to using "T" connectors to join the elements - but, again; what lengths and what impedance for each leg on the octopus?

I have TONS of Andrew coax tar to weatherproof the connectors - or would even be willing to buy some Scotchcoat to seal the junctions. But, with that much work and calculation; I'd only want to have to do it once. Especially once it's on top of my tower.

 

[talkpair]

Keep in mind that you can't solder aluminum. Whether it's an antenna element or shielding in a coaxial cable.

Whatever your plan of attack is, you will need to do the same thing to each element. I wouldn't mix elements from a DB224 either. From studying the harnesses of both these antennas, I get the feeling that the element on the DB304 is not the same as the element on the DB224.

At this point, I think you need to find an antenna analyzer and attempt to get a single element on frequency then measure it's impedance.

You'll need a 1/2 wave piece of cable for testing. At 147.08 MHz, this would be 40.125 inches multiplied by the velocity factor of the cable.
If the vP = 66%, then the length is about 26.5 inches. I would use a piece of good quality cable that hasn't been out in the weather.
Place a connector on one end of the cable that mates with the analyzer and prepare the opposite end to connect to an antenna element.

As for the harness......I would build one from scratch myself.
All segments on your existing harness are cut for another band split and are too short for 2 meters. Age and the lightning hit have most likely altered their original specs anyway.

 

[prcguy]

The mod to lower the dipole freq is easy but you can only take it so far, like you can't extend a 220MHz dipole with a bolt on each end to make it resonate properly on 2m. This is because the hot side of the dipole is actually a folded grounded monopole and if you extend it too far the overall length may be ok but the point where the element folds back on itself and grounds may not be ideal.

The grounded side of the dipole is simply a folded element grounded on both sides and you can treat that as a simple fat element. If your starting with one of the VHF high band versions then you will probably be ok with the bolt extension mod.

You can solder aluminum with aluminum solder. It works great but there is a fine line between melting the solder and the work if the work is very thin material. You could also cut the elements and extend with a small dia rod that is a snug fit inside the elements then solder or split the ends with a hacksaw and use hose clamps. It would not be pretty but you would be able to lengthen and shorten the elements easily to find the sweet spot.

A common spacing for dipole arrays is 3/4 wavelength between the feed points, or about 57 1/2" at 146.5MHz. Other spacings will work and you can look up assy manuals for various versions to see what they use.

talkpair's advice on tuning the individual elements with a 1/2 wavelength of coax (in coax with velocity factor) is good but I've had good and repeatable success using a random length of coax with a several inch string of 43 mix ferrite beads as a choke right at the dipole feed point.

I still think a home made phasing harness made from RG-6 or RG-11 using F connectors and F Tee adapters will be just fine or PL-259s will fit directly RG-11 or use RG-59 coax with UG-176 reducer like I did.
prcguy

OK. I'm still interested in the element mod. Thanks for the link! I'd seen several articles about that in the past, but my curiosity lies in the spacing between the tips of the elements. Stainless steel screws / bolts would be no problem to source. My hunch is that there needs to be a certain distance between the tops and bottoms of the elements that is carefully calculated. As I mentioned earlier; the space between the top to the bottom of the two bottom and two top elements is 28". The melon scratcher is; the distance between the top of the bottom 4 is only 24" from the bottom of the top four. I would have thought that all 4 elements would have been equally spaced. Any ideas on this oddity? What I'd like to do is to rotate the lowers at a 90 degree angle from the top, giving me a more Omni-directional radiation pattern.

In fact; I'll take it one better: "Suppose" I were able to find some aluminum tubing with the same diameter (inside and out), cut the ends off of the elements, insert some smaller copper tubing that would insert inside of the elements to stabilize them. Would the presence of the inner tubing inside the elements create another issue? Best case scenario: Find a couple of DB224's and cut the element ends off, leaving enough length to only have one weld joint at each attachment? OK, enough brain storming (or brain farting).

Then; I'm still left with trying to match a phasing harness. With the exception of dulling of the outer jacket of the phasing harness; it looks to be intact. Obviously; that's not the case. My hunch is that it took a lightning hit at some point in its life and the "opens" are inside the "Y" connections, which are covered by the thick black plastic covers. If I took a Dremel (or similar) tool and cut the plastic shields off of the junctions and only found that the solder joints were smoked; I could re-solder the connections - but at what lengths to match the antenna frequency? At the higher frequency, my guess is that they'd be too short to work with (enter wire stretcher) Worst case scenario: The coax is completely hosed and unusable. Back to square one. I'd have to build a harness from scratch. I'm not opposed to using "T" connectors to join the elements - but, again; what lengths and what impedance for each leg on the octopus?

I have TONS of Andrew coax tar to weatherproof the connectors - or would even be willing to buy some Scotchcoat to seal the junctions. But, with that much work and calculation; I'd only want to have to do it once. Especially once it's on top of my tower.

 

[PhotoGuyNJ]

WOW, awesome thread.. my head is spinning..lol.. thanks guys for all this knowledge & sharing

 

[nukesnooper]

Sorry for the long pause in responding. I didn't realize that it had rolled over to a second page and thought everyone had given up on it. I just noticed it today and read all of the responses.

So; it's not looking too good as far as making this thing holy. At least from my level of expertise and/or resources. I don't have an analyzer, so I'm not sure how to check the match.

I have extensive metal fabricating machinery at work in the machine / welding shop and could get them to extend the elements with donor elements from another antenna. I've checked my DB224 and the element diameter and ID spacing is identical. So; I could cut the elements and extend them - even make it almost unnoticeable - especially at 58' in the air. The issue at hand is: 1) What distance spacing between the element ends from one another, and 2) I'm not sure I understand as far as to whether I need to add to the bottom, top, or both. Forgive my ignorance. My thought is that I'd need to add a little to the top and a little to the bottom.

If I can get that much cleared up; I'll take a stab at the wiring harness.

So, the question of the moment is: What are the ideal physical dimensions for the elements? Again; I can either go the "donor" route, or even buy some aluminum tubing and have one of our machine shop guys fabricate the elements and do any aluminum welding necessary.

After crossing that bridge - assuming that I can - then I can concentrate on the harness. But, that's another can of worms.

Again; sorry that I didn't notice that the post had generated enough interest to roll over to a second page until this morning. I certainly meant no disrespect, and greatly appreciate al of your expertise and input.

Thanks!

 

[nukesnooper]

OK. Looks like I missed the boat. Off to the recycler.

Thanks to all that helped.

 

[mancow]

Are you sure you need to mod it? I've found them to work well enough as is for 2 meters.

 

[nukesnooper]

I know the wiring harness needs to be rebuilt, but have had differing opinions on the best way to do it. You've got to bear in mind that I'm a complete idiot as far as trying to match an antenna; much less attempting to match the elements to the proper Q. My thought was that all I really needed to do was to rebuild the wiring harness and that the elements would suffice - albeit; not perfect by electrical standards. But; would have still expected excellent performance, since I'd previously used the DB204 which was cut for the 155.000 range with superb performance.

I missed the boat when this turned into a 2 page post and didn't realize it for several weeks. I thought everyone had lost complete interest. I'd still like to rebuild it; but funds are scarce, and testing equipment is practically non-existent. I'd like to do any mods that are reasonable that will give me the best bang for the buck, while still providing the best performance. Yeah; that's everybody's story...

I've read a number of your posts in this and other forums and highly respect your opinion. If you have any suggestions to save this great beast from the scrap heap, it will be greatly appreciated.

 

[mancow]

Is there anyone you can beg, borrow, steal an MFJ or similar analyzer from?

If you end up wanting to get rid of it I would be interested in the elements. I have one of these antennas and would like to have the spare parts, or maybe I would get around to rebuilding it as well.

 

[nukesnooper]

Yes; and unfortunately, aside from my Motorola and Bird watt meters, that's the only test equipment in the area - and I'd still have to verify that he still has it. We used it when my repeater was on the sheriff's tower.

Honestly; I live in a small town in the middle of nowhere with only 8 or so licensed Hams. This doesn't even pass the sanity test to put so much effort into it if I hadn't spent so much time and effort to build a bulletproof (west Texas wind) tower. That's the main reason I want so much coverage.

I'd hate to see it go - especially to a scrap heap, but unless I can rebuild it - and I certainly don't lack the ability to do that - (just the knowledge) - there'd have to be one charming offer to ship the parts to Kansas. I'd much rather put it to good use, and have plenty of fabrication resources at work, but no RF engineers.