Continuity of antennas

[Flameout]

I was curious about a couple of mobile mag mount antennas. I recently purchased a Kenwood TK880 from ebay and it came with an antenna. I just figured it was a junk antenna, but it was this one https://www.mobilemark.com/product/a1845-series/

The actual antenna, that goes into the mount has a sticker 450-470 but testing SWR it is well over 3.5:1. I've had luck in the past using brazing rods, so I tried it and SWR dropped way down, to about 1.7:1.  Testing with a continuity tester, there is continuity between the center pin of the PL-259 and the outer screw on part. If I take off the antenna, there is no continuity, so I assume the base is good. When I check the actual antenna though (photo attached) there is continuity between the center pin and the screw on part. Is that normal? I know my Midland MXTA26 isn't like that

[axorlov]

Base-loaded antennas may have a DC connection between the center conductor and the shield. I would not discard mobile antenna for that.

Most interesting part is about you trying out brazing rods. Can you elaborate? I presume that was for the ground plane. How the rods were attached? How many of them? How long are they? What about the regular metal car roof, did you try to mount the antenna on it? SWR 1.7:1 is fine, and my question is: how different the mount between SWR 3.5:1 and SWR 1.7:1?

[SteveShannon]

51 minutes ago, Flameout said:

I was curious about a couple of mobile mag mount antennas. I recently purchased a Kenwood TK880 from ebay and it came with an antenna. I just figured it was a junk antenna, but it was this one https://www.mobilemark.com/product/a1845-series/

The actual antenna, that goes into the mount has a sticker 450-470 but testing SWR it is well over 3.5:1. I've had luck in the past using brazing rods, so I tried it and SWR dropped way down, to about 1.7:1.  Testing with a continuity tester, there is continuity between the center pin of the PL-259 and the outer screw on part. If I take off the antenna, there is no continuity, so I assume the base is good. When I check the actual antenna though (photo attached) there is continuity between the center pin and the screw on part. Is that normal? I know my Midland MXTA26 isn't like that

I’m curious how you tested the SWR.  Did you sweep the entire range you are interested in?  It’s hard to believe that it was one static value for the entire 106 MHz range the antenna is labeled for.

As Axorlov replied, DC continuity isn’t necessarily a sign that the antenna is no good.  Even an SWR value measured with a meter can fail to fully reflect the characteristics of the antenna.  You say there’s continuity, but understand that an ohmmeter measures DC resistance (or inversely continuity) which is completely different than RF impedance.  Just out of curiosity (because I’m still learning by asking questions that may or may not be dumb) what was the DC impedance you measured? Did it change over time?

[Flameout]

3 hours ago, axorlov said:

Base-loaded antennas may have a DC connection between the center conductor and the shield. I would not discard mobile antenna for that.

Most interesting part is about you trying out brazing rods. Can you elaborate? I presume that was for the ground plane. How the rods were attached? How many of them? How long are they? What about the regular metal car roof, did you try to mount the antenna on it? SWR 1.7:1 is fine, and my question is: how different the mount between SWR 3.5:1 and SWR 1.7:1?

I took out the antenna part from the base and inserted brazing rod. I cut it down till SWR is good. Checking SWR using a Comet CCA-500 set at around 465.000. I did that with a Nagoya UT72 (I think that's the model) and found that a piece of rod cut to about 4" has best SWR. All testing was done with antenna in center of car roof 

[axorlov]

Not clear to me. You say you unscrewed antenna and stick brazing rod into the center connector of the mount, as a vertical element? How you would do that? NMO mounts do not have a hole to insert a rod.

[Flameout]

2 hours ago, axorlov said:

Not clear to me. You say you unscrewed antenna and stick brazing rod into the center connector of the mount, as a vertical element? How you would do that? NMO mounts do not have a hole to insert a rod.

The top part of the antenna can be removed and it is the exact diameter of brazing rods that I have. They fit right in and can trimmed to achieve a good swr

[axorlov]

Ok, it is clear now. What you've done is changed the two 5/8 radiating elements for one 1/4 radiating element.

This is what I found: https://www.mobilemarkantennas.com/a1845 Apparently, there are five versions of this antenna tuned to different frequencies. Any chance of finding which version do you have? Looks like not the -50. Finding that would help with choosing length of the elements  (trimming or extending).

[Newb]

What is the length of the bottom rod measured from the bottom of the center phasing coil to the bottom of the rod when removed from the bottom loading coil?