Request info/clarification on stick vs folded dipole impedance

[SkepticalEd]

Hi all. While making my omni-directional vertical FM broadcast band antenna from the end section of a Winegard antenna (the unused VHF-Lo part which I cut to 97 MHz), I read that stick dipoles have a native impedance of about 75 ohms, while a folded dipole has an impedance of about 300 ohms. Is this correct? I took a 300 to 75 ohm balun and modified it to become an F female to twin lead adapter, just making a cleaner junction than taking the RG6 center conductor to one leg of the dipole and the shield to the other. I'm also asking because I've seen the same design from Winegard and now from Channel Master (except they design it to be horizontal, with Winegard using an additional opposed dipole), and both are using 300 to 75 ohm baluns. Which is correct? Is the stick dipole 75 (ish) ohms, or 300?

Thanks for any info.
Ed

 

[prcguy]

A folded half wave dipole is around 300 ohms depending slightly on element thickness and spacing and distance to the ground or large conductive objects. A center fed dipole is about 72 ohms in free space but lower impedance close to the ground or large conductive objects.

 

[SkepticalEd]

Thanks for the confirmation; however, continuing to do more reading, I came across something I'd seen several years ago while putting up my diy dipole, that, the dipole being balanced and the coax being unbalanced, that it needed a balun, but being as 1:1 baluns are not in my parts bin, I tried a 4:1 today, and picked up a NOAA transmitter several hundred miles away a little bit better using the balun than without. I just guessed that since I use 75 ohm feed line, that a dipole with 70ish ohms was fine, but it actually performs with a 300-75 balun. Regarding the 75 ohm feed line, I've got several hundred yards of RG11 and RG6 TS and QS each, and loads of compression connectors and the tools, so I'm going to use them, regardless of the minor loss. Still debating putting up my mobile to ground plane kit for my Maxrad VHF/UHF dual band antenna, or just buying a Diamond 130 or a Kreco, but the only stuff I listen to in my area is 118 to 470, so, think the discone's bandwidth might be unnecessary.

 

[paulears]

I did without any form of antenna analyser for years, but now I have them, I'm amazed when you can see an antennas output, and how this changes with positioning, height and obstacles. The number of digital always on transmissions in every band make so much of the old theory make sense - once you can see it. I've got an interest in VHF marine and have a number of different types that I sell. All with identical paper specs. Plonk them on an analyser and your suspicions, from the appearance that they are not the same are quite plain. One that I bought with the intention of using on land, as AIS data antennas was a poor performer up at 162, but good at 156 - which isn't very far, is it? I'm guessing it is a centre fed sleeve dipole inside the shroud, but the mounting hardware at the bottom messes up the performance - sticking one up on a timber batten with gaffer tape restores the 162 performance. Swapping baluns and different feeder impedances messes up the performance, but we're not sure how or why it happens in the individual cases. I tried a simple SDR receiver mounted to the antenna - no feeder, and like a proper analyser, you can see the results. Well worth getting one of these for antenna investigations. If you see known signals close or far from the desired one changing - then you know something is actually happening. S meters really tell you bugger all. Sometimes a drop in S meter can be an improvement in an antenna you need to cover a range of frequencies with.

 

[Ubbe]

..... but the only stuff I listen to in my area is 118 to 470, so, think the discone's bandwidth might be unnecessary.

That's the frequency range of a normal size discone. It will perform poorly if going higher in frequency. There's no other antenna, except log-periodic but that will be directional, that covers that whole frequency range. It's a different matter if you only need parts of the range, 118-137 and 155-170 and 420-470 as those can be covered by one separate antenna for each band that performs better, if you need to monitor weak signals that the discone can't pick up. But then you'll need to combine the antennas in some way, or the best solution are to use one receiver for each antenna.

If all your weak signals are in one general direction, like a half circle, then a log-periodic antenna will have some 5dB-6dB gain. Strong signals will usually not be a problem to receive from any direction using an antenna like a log-periodic.

/Ubbe

 

[SkepticalEd]

Thanks all for the info. I do have an old MaxRad VHF/UHF coupler, and a MaxRad VHF/UHF dual-band antenna, that was formerly used on two separate radios, but of course I can reverse it and use two separate, tuned antennas (wideband VHF and UHF) on one radio, though 118-137 band wouldn't be optimal, though I'm only concentrating on local rotor traffic, so I guess that's acceptable. Thanks all for the input.