Yagi Folded Dipole

[NJ-Radio]

I recently made my own yagi for the UHF 470-480Mhz range for monitoring NYPD from South Jersey.
It works rather well and the construction came out pretty good but I have a couple of questions for the experts since several frequencies are not as strong as they could be.

I used elements from an old FM antenna which were 6mm hollow aluminum. My folded dipole is made from heavy duty house wiring wire which is only about 2.5mm. Am I losing gain by using folded dipole wire that is less than my elements? Should I try using leftover elements

My second question is that I used a surface mount type female PL259 connected to the folded dipole (one end to the center, other end to the ground side). I then used spacers and screwed them into my squared aluminum boom. After I put the antenna up, I realized that I may have grounded to the boom since the screws were metal. Did I lose gain here as well?

Any tip or advice on maximizing my gain with this setup would be much appreciated.

BTW: Heres some info on my setup if needed:
Radio is an Icom R8500
Using homemade Yagi cut for frequency of 480Mhz
Also using Ramsey Broadband RF Amplifier (added great sensitivity to radio)


Brian

[ampulman]

Wow, monitoring NYPD from South Jersey. I'm in SJ, also (Voorhees).

Seems like quite a stretch. Where are you?

Hope you get some useful information; I'd like to try doing it too.

Amp

[NJ-Radio]

Originally Posted by ampulman

Wow, monitoring NYPD from South Jersey. I'm in SJ, also (Voorhees).

Seems like quite a stretch. Where are you?

Hope you get some useful information; I'd like to try doing it too.

Amp

I'm in Toms River and very close to the water. I'm only 17 feet above sea level (as per Google maps) but
being by the water seems to help. I love monitoring NYPD. I even have the record output of my Icom going into the computer with a program to record. When there's a foot chase, I just hit my record button. I have a few recordings already.

[specman]

I suspect you have a bad match at the folded dipole since you attached the co-ax directly to the element. A folded dipole has a characteristic impedance of around 300 ohms at resonance, so a balun would work well here. You could use one of those outdoor tv 300 ohm balanced to 75 ohm unbalanced to good effect here, and then use something like RG-6 down to your scanner.

[NJ-Radio]

Originally Posted by specman

I suspect you have a bad match at the folded dipole since you attached the co-ax directly to the element. A folded dipole has a characteristic impedance of around 300 ohms at resonance, so a balun would work well here. You could use one of those outdoor tv 300 ohm balanced to 75 ohm unbalanced to good effect here, and then use something like RG-6 down to your scanner.

I have LMR400 ran up to the Yagi - can't get much better than that. I'm confused on how to fabricate a balun to mate with my coax though. Do I really need one? Am I really losing gain? I do afterall, have reception - just want to maximize the gains.


Thanks

Brian

[specman]

Originally Posted by NJ-Radio

I have LMR400 ran up to the Yagi - can't get much better than that.

LMR400 is great coax, however in this case you're attempting to connect a 300 ohm impedance to a 50 ohm (unbalanced) line. I suspect this will affect your setup's performance.

If you've already pointed the antenna correctly and placed it high enough than this issue will likely need to be addressed.

Just curious; why did you elect to make a folded diplole design? A non-folded dipole would have been a better match for your coax direct feed.

[NJ-Radio]

Originally Posted by specman

LMR400 is great coax, however in this case you're attempting to connect a 300 ohm impedance to a 50 ohm (unbalanced) line. I suspect this will affect your setup's performance.

If you've already pointed the antenna correctly and placed it high enough than this issue will likely need to be addressed.

Just curious; why did you elect to make a folded diplole design? A non-folded dipole would have been a better match for your coax direct feed.

On my first antenna I did the split driven element with the coax tied to each side but since each element half didn't go straight through the boom, it became a challenge to have the elements stay securely in place. I just didn't care for that kind of design. Using the folded dipole I had a much better way to secure it. I also got the idea for this from http://www.arrl.org/qst/2006/08/KLITZING.pdf which is a nice UHF Yagi design from a 2006 ARRL article. Now, in that article they did describe making a balun with coax but when I tried it, the reception was really poor. Once I cut off the balun, the reception was MUCH better. I didn't have the UT-141 coax they used but had some (I think) RG58. Maybe that was the issue, not sure.
The antenna is definitely working though and it is directional - if I rotate it away from NYC, even slightly, I will lose my signals so it seems to be performing with the narrow beamwidth associated with Yagi's.
But, as you pointed out, I'm aware that I may not be properly matching impedance so I'm not sure if because of that I'm lacking some gain that can be claimed from a better design.


Thanks

Brian

[specman]

Originally Posted by NJ-Radio

But, as you pointed out, I'm aware that I may not be properly matching impedance so I'm not sure if because of that I'm lacking some gain that can be claimed from a better design.

Hi Brian,

Thanks for the article. I think the balun described in the article would be difficult to construct with anything other than the material mentioned.

Baluns get really tricky at UHF and designs you can find easilly for HF will not work well at all. I'm thinking that a "gamma match" might actually be better in this case however it might be difficult to design one for your needs.

Personally I would abandon the folded dipole in favor of a regular dipole; there's some tricks you can use to handle the construction issues.

Good luck!

[NJ-Radio]

Originally Posted by specman

Hi Brian,

Thanks for the article. I think the balun described in the article would be difficult to construct with anything other than the material mentioned.

Baluns get really tricky at UHF and designs you can find easilly for HF will not work well at all. I'm thinking that a "gamma match" might actually be better in this case however it might be difficult to design one for your needs.

Personally I would abandon the folded dipole in favor of a regular dipole; there's some tricks you can use to handle the construction issues.

Good luck!

No problem Specman - I'm glad to share it.

What would be the main symptom of an impedance mismatch, in regards to receiving only?
In other words, would reception be extremely poor, non existent or would you just lose some gain?
I know that SWR is a concern in transmitting but I assume for only receiving it's not an issue - or is it?

Thanks

Brian

[specman]

Originally Posted by NJ-Radio

What would be the main symptom of an impedance mismatch, in regards to receiving only? In other words, would reception be extremely poor, non existent or would you just lose some gain? I know that SWR is a concern in transmitting but I assume for only receiving it's not an issue - or is it?

We normally only refer to SWR in regards to a transmitter setup. In a receiver setup a significant impedance mismatch will translate to a reduction in performance. In this case the outer shield of your coax is actually now part of your antenna, meaning that the corresponding design frequency is out the window and the system will resonate on something other than what you had intended.

So to answer more specifically you are noticing that the antenna DOES work, however the performance is not as good as expected. Would correcting the mismatch improve things? I believe it would, however like I've said we can't discount other factors such as distance, topography etc. between you and the transmitter sites. I guess we would need another reference antenna to determine the performance of this antenna. Some folks will argue that a match isn't all that critical and can back that up with thier own personal experiences. Have you ever used anything else that you can compare with ie. a disone?

[NJ-Radio]

Okay - today I removed the metal screws and spacers from the SO239 connector, no longer grounding to the boom. Surprisingly, there was no change at all in signal.
Tomorrow I'm going to try the split driven element design to see if that gives some more gain.
I'm trying to come up with ideas on how to do this without drilling into the boom in case I need to revert to my current folded dipole. Any ideas?
Also - I use the Yagi Calculator. Am I supposed to just cut the radiator element in half?
Should it be isolated from my metallic boom? Any differences if it goes into it? Recommended gap between the element halves and the boom?

Thank you

Brian