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  #31  
Old 02-26-2014, 05:40 PM
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Quote:
Originally Posted by DaveInDenver View Post
Not sure what that mass statement means, mass has nothing to do with anything. Surface area and lengths are critical. A 5 lb slug of metal under an antenna isn't going to work while a very light region of foil might, practically speaking.
You are correct. However, I interpreted his use of the word mass to be in the context of the amount of sheet metal for a mobile install. I deleted that quote in my post above and added a sheet metal comment to avoid confusion on the subject. Good point.

I got my Flexi-whip antenna today. I was looking over the provided cut chart and your previous posts about the mast length. If one assumes linearity over the short 5mhz increments (yes I know it is not linear but lacking more information...) I get 19.62 inches total mast length for 146mhz. This is a bit longer than what you have but is in line with what you said about starting at 19.5 or a bit longer. I can then try to find someone with an antenna analyzer and check it out. This antenna is very flexible but much thicker than my MFJ mini mag mount 1/4 wave.
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  #32  
Old 02-26-2014, 05:54 PM
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Can't believe I just cut it too short, 1/2 inch too short.

Had the 1/8" over the 19.5 stuck in my head and cut it to 19 1/8", which is about 149Mhz
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Last edited by CO Hunter; 02-26-2014 at 06:39 PM.
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  #33  
Old 02-26-2014, 06:48 PM
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How far off are you now? You can fix it by adding length electrically and it might be pretty easy, modify the transmission line between the NMO adapter and the antenna or maybe add some inductance at the base. I was wondering how it would go tuning with the dual SO239/PL259 discontinuities at each end of the feedline.
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  #34  
Old 02-26-2014, 09:41 PM
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I am off by 0.5 inches, too short. I understand the cut chart is for a "typical" installation such as a roof mount NMO. I understand adding the SO259/PL259 connectors will change things but I suspect not that much. Getting it tuned for my primary location is my concern at the moment as I will likely be running my 5/8 wave when secondary NMO location is in use. Maybe if I call Tessco and tell them what happened, just maybe I can get an idiot discount on another one...haha.

I was able to open the 2m Pikes Peak repeater on 5w but the return signal was noticeably weaker than normal and got a call back that my TX was 95% static and unreadable. I was able to open the nearby low azimuth 70cm repeater and got a strong return signal. I was on the run this evening so I didn't have time to get into a discussion with anyone about the audio quality on 70cm. The 2m signal seems to be a lot worse than what it should be for the amount of cutting error. Maybe I need to recheck that the antenna is seated securely, that gasket was very tight on the mount.

Maybe I should just weld it back together and recut...haha

As a side note, my current set up runs through an antenna switch.
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Last edited by CO Hunter; 02-26-2014 at 10:07 PM.
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  #35  
Old 02-26-2014, 10:13 PM
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Actually the UHF connector is a terrible discontinuity. Nominal impedance of 34 ohms and about 1.3:1 VSWR at 500MHz. Putting them at each end of a length of 50 ohm coax is a textbook transmission line reflection diagram problem. So what is the SWR right now? You could just add a short length of wire, maybe 3 or 4uH of inductance, to bring it back into resonance. That's the great thing about VHF and UHF, it won't take much to move things.
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  #36  
Old 02-27-2014, 08:57 PM
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So this is what I found. Apparently the gasket had caught a bit on the NMO mount, so I removed the gasket and reseated the antenna without it. I don't have a SWR meter for my TM D710 so I took my Ft857 and my little analog LDG meter and hooked those up in the 80. My signal report at 5w, on 2m into the Pikes Peak repeater was a little weaker than normal put clear with a SWR of 1.1 (did not move the needle). I tried the 70cm repeater at 2w and the meter showed an SWR of roughly 2.1-2.2. I did not get a signal report as the FT857 rolled back the power (did not open the repeater). I have seen this happen on 70cm in the past with SWR near this level but higher. I can reconnect my Kenwood tomorrow and try the 70cm repeater again. Just didn't feel like working on it more tonight.

Hopefully I can get this tuned back up for the area of 70cm I want. If not, no great loss but very annoying. How would I add the inductance back in? Very intrigued. I am guessing it does not involve welding ...
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  #37  
Old 02-28-2014, 05:56 AM
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Yeah, I know what you mean. I usually leave out the gasket when tuning the Flexiwhip. The Larsen NMO-Q is the same way, with a fixed center conductor. So I want to make sure the antenna is making solid contact with the base while tuning so I know it's good. Then I recheck after putting in the gasket and smearing some dielectric grease to make sure nothing has changed.

The difference between good and bad with that gasket is very slight. The generic rubber gaskets you find in hardware stores are usually too thick and don't compress enough, so be sure to use the one they include or one just like it. I also found that the hardware store plumbing gaskets deteriorate and turn to goo after a couple of years being exposed to UV and mag chloride...

I may have mentioned, but you have to interpret the meter readings. A 50Ω resistor has a prefect load match but radiates almost zero RF. So you may not have or even want to see a 1.0:1 SWR reading. My MFJ-269 only has I'm guessing a 5% precision, so 1.0:1~1.05:1 is what I read into a calibration load and precision gets worse at greater than 2:1 mismatches.

That 2.2:1 or so SWR does not surprise me, the bandwidth of a 3/4λ antenna is narrow, so if you've dialed it in for a 2m repeater on the high side it's quite likely you'll be on the long side of the curve for 70cm repeater inputs. See posts #21 and #24, when SWR at 147Mhz is 1.4:1 the same antenna is 2:1 at 440MHz. Those NEC models don't factor in parasitics or discontinuities inherent in a real antenna. Those are just feedpoint characteristics. We already know the UHF connector is lousy at UHF frequencies (remember 'UHF' when the PL259/SO239 was designed in WWII was what we now call VHF), it's likely the RG58 is start to get lossy at 440MHz, etc.

VK3JEG ran some tests on the UHF connector in fact: http://www.qsl.net/vk3jeg/pl259tst.html

I was suggesting you add a short length of wire, like 1/2", between the coax and antenna. That's not a practical solution, though. It's very common with mobile HF antennas to put coils at the base to change impedance because being physically short relative to wavelength yields extremely narrow bandwidths. So you'll be dead on in the phone spectrum but completely mismatched in the CW and data. So you add reactance to bring resonance down or up. You might also be able to add capacitance but it's easier to add inductance. I'll have to think about it, it might be possible to add a coil at the top end or maybe a parallel cap. Problem is mechanically how to make it work.
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Last edited by DaveInDenver; 02-28-2014 at 06:35 AM.
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  #38  
Old 02-28-2014, 09:29 PM
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I don't care for their gasket. Today I seated the gasket carefully against the roof and tightened the antenna. I could still get a piece of paper between the gasket and antenna base along most of the circumference. I like the Larsen gasket much better but the diameter is wrong for the flex-whip.

So I tried the 70cm band again but after a time or two the ft857 started getting out on 70cm with a SWR of about 1.25:1 ! I have found this radio to be a bit finicky on 70cm. The Pike Peak repeater (146.970) was still at 1.1:1 (or no needle movement). The 70cm repeaters I was hitting were 448.450 (Pikes Peak) and the Marc 448.650 (low azimuth). For these higher frequencies on the 70cm band the graphs on page 21 show a much lower SWR than 2:1. Interestingly, the length in that model was 19" only 1/8" less than what I cut mine to. Is it possible I actually got very lucky after all? Need to find someone local with an antenna analyzer.

I have used a mix of UHF connector manufacturers to build cables over the last few years. I use those cables for HF/VHF/UHF. Now I wonder what I have done...
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  #39  
Old 03-02-2014, 08:48 AM
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The weather has kept me from playing around this weekend. I posted the UHF SWR behavior in the FT857 yahoo group and got this response.

"Dear Sir: I repaired a radio with same similar symptoms. Get a Service Manual. Follow the RF trace from the VHF/UHF finals to two PIN Diodes leading out to the low pass filter section and from there going out to the antenna SO-239. In my case it was the first pin diode, which you can check from the ((((((circuit trace to the diode))))))) and then the diode itself. Mine was not soldered from the factory, but worked for some time before the symptoms appeared. When I touched it with the DVM probe the pin diode went flying over my head. It's a 25 watt pin diode, I replaced it with a 50 watt pin diode. Your SWR intermittent is caused by the Foward/Reflected sensor is seeing the loss of connection through the LPF, and antenna conn. Good Luck"

I have the circuit diagrams. This looks like it may be a bit above what I am able to do. I may open up the case and take a peak to see if I can identify the area of the PA circuit involved.
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  #40  
Old 03-02-2014, 11:07 AM
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He seems to be talking about D3021 & D3019 or maybe D3023 & D3039
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