Homebrewed Off-Center Fed Dipole question

Bellingham_Scanner · May 27, 2013

While looking for scanner antenna projects I ran across the Homebrewed Off-Center Fed Dipole Scanner Antenna here on RR.

My question is this design uses a 300 ohm to 75 ohm transformer and 75 ohm coax.

All my scanners have a 50 ohm antenna input.

Is the impedance mismatch not worth worrying about since I am not transmitting?

prcguy · May 27, 2013

The radio to coax impedance mismatch is not much to worry about when compared to the mediocre performance of this particular antenna.
prcguy

Bellingham_Scanner said:
While looking for scanner antenna projects I ran across the Homebrewed Off-Center Fed Dipole Scanner Antenna here on RR.

My question is this design uses a 300 ohm to 70 ohm transformer and 75 ohm coax.

All my scanners have a 50 ohm antenna input.

Is the impedance mismatch not worth worrying about since I am not transmitting?

searingxheretic · May 27, 2013

prcguy said:
The radio to coax impedance mismatch is not much to worry about when compared to the mediocre performance of this particular antenna.
prcguy

What makes you say mediocre? Lots have had success with this style of antenna.

Sent from my cm_tenderloin

LtDoc · May 27, 2013

An off center fed antenna is not a balanced antenna to start with so no balun is required if you are using coax for feed line. As for the that balun being an impedance transformer, I seriously doubt if that antenna is going to be near 50 ohms or 70 - 75 ohms if it's used on more than one band.
An off center fed antenna (OCF) works on a number of bands because it's harmonically related to it's primary resonant frequency (what it's cut for). Because of that, it won't ever be as efficient as a resonant antenna, but it will certainly work. As the frequency goes up from it's primary resonant point the radiation patter will change too. If the resulting pattern happens to cover a desired station then that's good. But the further you get away from it's primary frequency the more lobes and nulls are developed and who knows what the pattern will look like.
- 'Doc

Bellingham_Scanner · May 27, 2013

Thanks, Doc.

I am looking for a simple but wide-band antenna for an SDR using the Realtek TV dongle.

Mine came with a tiny mag mount antenna that is pretty poor even on local FM broadcasts. I have plans for mounting the SDR dongle with a Raspberry Pi running as an SDR server on my roof in a weather proof enclosure.

I guess I am most interested in standard VUF/UHF stuff plus anything else this cheap SDR can receive.

popnokick · May 27, 2013

"Mediocre" is relative. Compared to a narrow-bandwidth beam tuned to a small segment of the total spectrum your receiver covers... yes, it's mediocre. Compared to a discone mounted up high and outdoors... it's mediocre. But compared to a rubber duck antenna on a portable scanner, or a back-of-set antenna that came with your receiver, an OCF dipole is a major improvement for very little time, effort, and $$.
As to how they work with USB SDR dongles... I'll let you know soon.

dmg1969 · May 28, 2013

I built one and would call it anything but mediocre. I actually see very good results with it based on what I listen to.

prcguy said:
The radio to coax impedance mismatch is not much to worry about when compared to the mediocre performance of this particular antenna.
prcguy

popnokick · May 28, 2013

I just tried my home-brewed OCFD VHF/UHF homebrewed scanner antenna with a NooElec RTL820 SDR dongle. The USB dongle comes with a crummy little magnet mount mobile antenna that is essentially about a 5 inch piece of wire on a magnet base. I was getting nothing with it... not even the strongest FM broadcast stations that I receive. I thought the dongle was bad, so ordered another one... same thing... no signal. So instead I ordered a BNC to MCX pigtail adapter and disconnected the OCFD that is in my attic to the back of my "normal" scanner downstairs. I used the pigtail to connect the coax from the OCFD to the USB SDR stick. WOW! Night and day difference. Picking up New York Center ARTCC, lots of aircraft, all the FM stations, VHF high/NWS, and more. The spectrum display is really useful for monitoring large chunks of spectrum for new activity. If you see a new signal pop up just drag the tuner bar over to the peak/center freq and listen.

LtDoc · May 28, 2013

Performance is relative? Absolutely. It only depends on what you are make a comparison with.
- 'Doc

searingxheretic · May 28, 2013

popnokick said:
I just tried my home-brewed OCFD VHF/UHF homebrewed scanner antenna with a NooElec RTL820 SDR dongle. The USB dongle comes with a crummy little magnet mount mobile antenna that is essentially about a 5 inch piece of wire on a magnet base. I was getting nothing with it... not even the strongest FM broadcast stations that I receive. I thought the dongle was bad, so ordered another one... same thing... no signal. So instead I ordered a BNC to MCX pigtail adapter and disconnected the OCFD that is in my attic to the back of my "normal" scanner downstairs. I used the pigtail to connect the coax from the OCFD to the USB SDR stick. WOW! Night and day difference. Picking up New York Center ARTCC, lots of aircraft, all the FM stations, VHF high/NWS, and more. The spectrum display is really useful for monitoring large chunks of spectrum for new activity. If you see a new signal pop up just drag the tuner bar over to the peak/center freq and listen.

Out of curiosity, what frequency did you "cut" your OCFD to?

Sent from my cm_tenderloin

popnokick · May 28, 2013

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I built mine to "spec" as in the RR Antenna Wiki, I.e. one leg 18 inches and the other 48 inches. Based on what I've read here on RR, it's necessary to use 18 and 48 inches to achieve proper impedance match and greatest bandwidth. Mine also has about 35 feet of RG6 as the coax to my scanners.

n4yek · May 28, 2013

I built this antenna and it does a fine job receiving from 40 MHz to 800 MHz, does just as well as my ST-2 antenna.

nanZor · May 28, 2013

popnokick said:
Based on what I've read here on RR, it's necessary to use 18 and 48 inches to achieve proper impedance match and greatest bandwidth.

That's right. The actual first resonant frequency is about 88mhz. This could be a problem for those with an FM broadcast overload conditions.

Unlike an amateur OCFD, this one is not designed to provide the absolute best match on even related harmonics, but a "good enough" compromise all around, thus the 18 and 48 inch length. So impedance matching is not all that critical, although you definitely want the 300:75 (4:1 ratio) transformer as the actual antenna impedance varies up and down from about 100 to 500 ohms or so depending on frequency. A transformer with isolated windings is preferred.

It has been a fun antenna that has definite textbook problems, and has been criticized as such since the 80's, but is still worthwhile as a first venture. For some it may be all they need.

nanZor · May 28, 2013

LtDoc said:
...As for the that balun being an impedance transformer, I seriously doubt if that antenna is going to be near 50 ohms or 70 - 75 ohms if it's used on more than one band.

Bingo! For general purpose scanning over a wide spectrum, it varies from about 100 to 500Z, so the typical 4:1 tv-type transformer works ok in this application. The lengths selected were to provide this kind of coverage, rather than any optimum harmonic relationship.

Thing is, most tv-type transformers are also baluns, and in this case the antenna is so unsymetrical, and the chance of the actual balun being a voltage type and not a current type, means that the braid will be part of the antenna. A transformer with an isolated winding is preferred, although this does not guarantee non-interaction with the braid.

Some may put ferrite chokes on the feedline, or some may actually find the braid interaction useful on VHF-low.

Certainly it is a compromise, but in the time I've taken to write about it, I could have built one and then thrown it away - or not.

LtDoc · May 28, 2013

18" + 48" = 66". Convert those inches to feet and you have 5.5 feet. Divide 468 by 5.5 feet and you end up with about 85 Mhz as that antenna's 'design' length and frequency. Allowing for the normal discrepancies, call it for the bottom of the FM broadcast band. It should do pretty good there, and on multiples of 88 Mhz, sort of. Lots of variables in that, where the antenna is placed, environment, color socks you're wearing too. Without actuallyt measuring it (antenna analyzer) I'd have to say it should 'work'. Probably better than a rubberducky in most cases. Impedance matching, or how well your radio 'likes' it? That's part of that measuring thingy, and I very much doubt if it's going to come out 'good' in most cases. Luckily, most receivers don't make a big deal about impedance matching. But, matching impedances, if possible/practical is a very good idea. Sometimes it's worth the effort, but not all of the time. That get's into that 'relative' thingy and cost.
Then you get into the shape of the antenna's radiation pattern at different frequencies. That 'shape' will change, sometimes for the better, sometimes not. Good luck with that. If you are determined to have a multiband antenna with gain on almost all bands then I think I'd start looking at a log-periodic type beam. Oh boy, will that get to be 'involved'! Or maybe a stacked array type antenna for each band of interest? And the best 'improvement' possible for any VHF/UHF antenna is height.
An OCF antenna does work, but it's not a miracle worker...
- 'Doc

nanZor · May 28, 2013

LtDoc said:
... Then you get into the shape of the antenna's radiation pattern at different frequencies. That 'shape' will change, sometimes for the better, sometimes not.

Precisely - at 800mhz these ocfd's are looking nearly straight up since they are reeeallly long wires at those freqs. But if you are in a needle-bending rf zone, it may not matter, or if you are lucky with a reflection perhaps indoors.

Even at 450 mhz, we're looking at basically a cloverleaf pattern and not something looking straight outwards.

Yet even with all this, the intrepid experimenter might find that cutting the element lengths in half, that is only 2-feet on one side, and only 9 inches on the other (with the longer side facing upwards) will improve the shape a bit especially on mil-air. Still cloverleaf, but better than looking down.

Is it a DPD production mil-air competitor? Hardly.

And for many scanners with poor front-ends with a tendency to overload, the non optimal look angles may not be a problem either. It really is a case of cut-n-try knowing that it isn't the "optimal" antenna.

popnokick · May 29, 2013

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If cost to make and try it is a factor for anyone needing a better antenna, then the OCFD produces a very high return on the dollar. $10 in parts and 30 minutes of your time. And as was noted in this thread, if it doesn't work as you expected, it's an easy "Oh well". But it WILL outperform a telescoping whip or rubber duck if properly assembled.

nanZor · May 30, 2013

Half-length OCFD

This kind of illustrates that impedance match is not the only criteria. Milair fans may especially want to try their hand at the 24-inch / 9-inch smaller version. The smaller version has a better angle, but obviously not as good as a dipole...

Here is the original long version in free space at 300 mhz with 4-foot / 18-inch legs. Note that maximum gain is downwards.

popnokick · May 30, 2013

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Gain!? Surprises me a little, I thought like any dipole an OCFD exhibited 0 gain?

nanZor · May 30, 2013

Here is the short version ocfd at 300mhz with 2-foot / 9-inch legs:

Showing the classic cloverleaf, but better than the original!

Heh, don't even ask to see what 860mhz looks like.

Homebrewed Off-Center Fed Dipole question

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