Help with short dual band antenna repair

[Yuccahead]

Need some help on an antenna question.

I have a Diamond AZ504 antenna. It's basically a short dual band antenna I was trying around town so I didn't have to take off my antenna every time I went into my garage. It turned out to be just a little too long and I have replaced it with a Comet B-10. However, I was keeping the AZ505 under my rear seat as a back-up. I've discovered this isn't the best idea. The antenna ended up between the folding seat supports and was snapped in half when I folded the seat down.

So now I have a perfectly good base to this antenna and a shaft that has broken under the insulation and outer coating. I have disassembled it a bit and thought, why can't I just replace the shaft of the antenna. The original shaft is seems to be a 1CM tall and 3mm thick brass sleeve/base perhaps coated to provide a better contact. The rest is a thinner metal rod (say 1mm) surrounded by some 1mm thick white plastic insulation and then an outer black rubber sleeve.

To try to salvage the ridiculous cost of these antennas and give myself another backup, I'm wondering if I just replace the shaft with an equal length of steel (or whatever metal) rod and heat-shrink tubing, will I still have a working antenna with roughly the usual 50 Ohms of impedance? Or do I need something closer the thinner rod (wrapped in the plastic) to make this thing work somewhat like it did before.

This would be a third antenna so I'm not too concerned if it doesn't work perfectly. But I don't wan't to fry my mobile radio either. It would just be something to keep in the Jeep and use only if I manage to snap off my other 2 off on trees or garage doors.

Thanks.

David
(KF5QDM)

 

[4x4junkie]

If the replacement rod is the same length as the original, I see no reason why it can't work (rod thickness also matters much less than it's length does)... I would suggest you connect a SWR meter to it just to verify it actually is working though (verify that you got the length correct).

 

[Yuccahead]

Thanks 4x4 Junkie.

David

 

[Tennmogger]

David, I looked up the AZ504 because I have never used one. Since this is a dual band antenna, and is about 15.6 inches long, Diamond undoubtedly uses matching networks inside the base. If that got busted, you will not have a dual band antenna any longer if you just replace the 'wire'. With a length of 15.6 inches, they have some inductance in series to make up for the shortness on 2m. A 2m antenna should be around 19" for a quarter wave. On 70cm, they probably treat the 15.6 inches as an end-fed half wave, again requiring significant matching.

If the matching networks are destroyed, you can't just replace the wire. What you can do is modify that antenna base to a single band antenna for either band. You would need to make a direct connection from the center conductor of the coax connector to the end of the wire, and trim it as a quarterwave antenna for either band. Aim for about 19" or about 7 inches depending on which band you want, and trim for best SWR.

Piano wire makes a tough antenna if you can solder it.

Bob

 

[Yuccahead]

Thanks very much for the additional info Bob.

If I'm following you correctly, I'm pretty sure the base was un-damaged. I just used my Jeep's seat supports like a giant pair of scissors to break the plastic that surrounds the actual antenna wire. The base wasn't touched. To remove the original shaft, I only loosened the set-screw in the base and just pulled the shaft out. After my first post, I conducted a more detailed autopsy and found the thing was a rubber outer coating over a 2-3 mm plastic tube that surrounded a thin (.5mm?) braided copper wire. At the bottom of the shaft, Diamond installed a brass sleeve that fit over the plastic and was soldered at the bottom to electrically connect the sleeve to the center antenna wire. I assume the signal was carried to the sleeve by the base that surrounds the bottom 1/2" or so of the base. I ended up desoldering the sleeve from the original shaft and soldering it back on to my replacement steel rod. Above the brass sleeve, I covered the shaft with heat shrink and the original shaft's rubber tip. I'll try to put a SWR meter on it but I'll have to track one down first. I feel silly blowing $50 for something I'm going to use very infrequently (and cost more than the antenna I'm going to test).

David

 

[Tennmogger]

Great investigative job David. Most radios today have SWR protection so you are probably ok to try it. Some radios have an output meter that acts as a tattletale for SWR. As SWR gets worse, the level on the output meter will reduce due to power reduction. You can use that as a rudimentary SWR indicator by transmitting on frequencies across the band, especially top and bottom, and observing relative meter readings. For example, if the power indication is better at the top of the band than at the low end, then the antenna is short for the band. Lengthening the antenna will move the best frequency response downward, etc. With a good antenna, the power meter should maximize somewhere within the band.

Bob