My homemade Off Center Dipole Antenna

[kandrey89]

I have a question:
I am trying out figure out how to build a Guanella BALUN to do the 4:1 conversion instead of the ****ty $5 converter, and was wondering:
Suppose I have an OCFD that matches 42MHz as the lowest optimal frequency. Now I need to add a BALUN.
From what I understand, adding inductance to an antenna is like base loading it, allowing you to receive higher bands but at a weaker signal, less current would pass because of the inductor. Would this shift my optimum 42MHz reception? However I need some turns on the toroid in order for it to work properly as a BALUN. So how much inductance is enough for the Guanella BALUN and not enough to cause reception issues?

 

[br0adband]

While I'm not expert with this antenna stuff, it seems pretty logical to me based on all the research that I've done and all the posts people of all backgrounds have made about the OCFD that the reason for the balun/transfomer as part of this whole circuit is simply to match the antenna to the feed line going to the receiver. I can't imagine there's going to be some magical balun or transformer that really improves the reception performance overall - since this is a receive only type antenna there's no reason to get that deep into impedance matching or SWR stuff or anything of the like but, again, that's just my opinion on it.

I ended up choosing the better 300-to-75 ohm transformer from Radio Shack as they had three different ones at one point and after some of that research I mentioned I believe it was hertzian (a member here at RR) who posted that in his testing the better quality transformer seemed to work better for him in his usage. The one I'm referring to is this one:

https://www.radioshack.com/collecti...tdoor-matching-transformer?variant=5717015365

Haven't had any issues with it since I put my OCFD back together for a second time earlier this year and it works great for me in my situation. I believe - and this is just my own opinion - that one of the great things about the OCFD is the overall simplicity of it and just how well that turns out to work in real-world usage. Two pipes or pieces of wire cut to length, attached to the balun/transformer and fed to a receiver and you're pretty much done.

That's not to say that a project of making your own balun is useless, of course, but in some respects it does seem like a bit of overthinking the whole idea, maybe?

 

[HelixArray]

What is the consensus on the RR transformers..that they used better components or shielding to the forks? I was wondering what the leakage would be so I grabbed the shortest leads I saw on amazon. Are they even shielded or does it not matter so much?

 

[kandrey89]

The pigtails coming out of 4:1 transformer bought on amazon/ebay, are what's called twinax, they don't have shield, and they are called balanced. The antenna is also balanced, so by connecting a balanced line from the transformer to the antenna you're doing exactly what you need to do.
It would be incorrect to replace the pigtails with a coax cable and connect the coax to the balanced antenna.
https://en.wikipedia.org/wiki/Balanced_line
https://en.wikipedia.org/wiki/Unbalanced_line

Coax cable has unequal impedances with respect to ground, that's why it's unbalanced.

 

[prcguy]

The wires coming off a 75 to 300 ohm is not called "twinax", that's a specific type of coax with two individual and parallel center conductors inside a shield, with a whole line of coaxial type connectors to match.

The Off Center Fed Dipole is also not a balanced antenna, its very unbalanced with unequal currents at the feedpoint but it does benefit from the "balun" effect of a 4:1 transformer. The feedpoint of the OFC is not really 200 ohms or 300 ohms as reported, its all over the place on the scanner version people are building here.

When an OFC is designed for a certain frequency and used on harmonically related bands it does match ok to around 200 ohms on those specific harmonic bands but the scanner version is simply some random lengths of whatever material you make it from and can't really be compared to an off center fed "Carolina Windom" which it has some resemblance to.

I think some of the discussions of what material to make it from and how thick are almost completely irrelevant because this particular antenna is a random meaningless length on all but a few discreet frequencies. VHF low band will suffer the worst due to its length and the fact most TV transformers start dropping off in performance below 50MHz. I've measured a few TV transformers back to back with lots of loss in the VHF/UHF region and I seem to remember 3 to 5dB (half that for one) was common for some brands.

The OFC is a cute little project that many people will benefit from compared to smaller or indoor antenna, but its not a mystery or magic antenna that some are touting it to be.
prcguy

The pigtails coming out of 4:1 transformer bought on amazon/ebay, are what's called twinax, they don't have shield, and they are called balanced. The antenna is also balanced, so by connecting a balanced line from the transformer to the antenna you're doing exactly what you need to do.
It would be incorrect to replace the pigtails with a coax cable and connect the coax to the balanced antenna.
https://en.wikipedia.org/wiki/Balanced_line
https://en.wikipedia.org/wiki/Unbalanced_line

Coax cable has unequal impedances with respect to ground, that's why it's unbalanced.

 

[popnokick]

Perhaps this is a good time to summarize what the Homebrew OCFD Scanner Antenna IS and IS NOT.
The home-brew OCFD for scanner reception as shown in the RR Wiki IS:
- Simple to build with common parts available in any home or hardware store
- Very low cost
- Easily concealed in an attic, behind a drape, next to a window, or hanging on a balcony
- Portable, and easily rolled up and used while traveling with a scanner
- A MAJOR improvement over a rubber attenuator.... err, rather rubber duck antenna or back-of-set telescoping antenna
- Effective as a RECEIVING antenna for a wide range of common scanner frequencies
- Exhibits some gain over a dipole on some frequencies
- Well suited to hobbyist scanner listening or when the need for a quick, ad hoc scanning antenna arises
The home-brew scanner OCFD antenna is NOT:
- A professional grade antenna suitable for tower mounting or commercial installation
- NOT comparable to a high-gain purpose built directional or omnidirectional antenna for specific frequencies
- NOT as effective below 50 mHz and above 450 mHz as other antennas can be (although in comparison to a rubber attenu... antenna, VERY effective

 

[majoco]

What prcguy said:

The Off Center Fed Dipole is also not a balanced antenna, its very unbalanced with unequal currents at the feedpoint but it does benefit from the "balun" effect of a 4:1 transformer. The feedpoint of the OFC is not really 200 ohms or 300 ohms as reported, its all over the place on the scanner version people are building here.

+1.

You can't quote a specific transformer ratio to match any OCFD (HF, VHF or otherwise) as it is configured here as a broadband antenna and so has an undefined impedance at any frequency. The ham band HF version has dimensions chosen so that somewhere in those bands the impedance is reasonably the same (as the bands are harmonically related in frequency) and so a simple transformer can be used. But to try and make a VHF/UHF OCFD with a known impedance? Forget it. All you can do is 'assume' (careful!) that it will generally be somewhere between 100 and 600 ohms and a 4:1 to 9:1 transformer will work - even so there will be some "deaf spots".

 

[popnokick]

What would be interesting to see is an SWR plot from an antenna analyzer like the Comet CAA-500... showing the "sweet spots" of resonance across say, 50 mHz to 500 mHz of the OCFD. If I can borrow one and connect it to my OCFD in the attic, I will do so. But it may be awhile, so if someone else has a suitable plotting analyzer.... please post! Typically, an HF OCFD will have multiple resonances across a wide range of frequencies.... that is the whole point of the design. And I know that HF OCFD antennas change impedance as you go higher above the ground, and require different ratios of balun transformers if you mount them above about 50 feet. I'm assuming something similar happens with the VHF OCFD (?) At the very least, it would be interesting to run the plot with the analyzer, then move the antenna or change the feedline length.... rerun the analyzer plot and observe any changes.

 

[prcguy]

I have a CAA-500 and would be happy to loan it but your 3,000 mi away. The scanner OCFD is usually mounted less than a foot away from a mast with one element running parallel to the mast and feedline, so that has some effect on the impedance like an HF version 30ft off the ground.

I have several HF OCFDs, 80m on up and 40m on up versions and they work very well because most of the HF ham bands are harmonically related and fairly narrow band, so it works out well for that design. They work on the fundamental and even harmonic bands based on the lowest design frequency, so a 7MHz version works great on 14MHZ and 28MHz then everywhere else its just a random length of wire.

An OCFD designed for 155MHz would hit an odd harmonic at 465MHz where it would not present a good match and would skip over the 800MHz band and land on 930MHz. It would have a good match at 310 and 620MHz and so on, but that's not useful for scanners.

One of the things I don't like about the OCFD for scanner use is the gain lobes at steep angles up and down with a null at the horizon. This is ok for an HF version where you can make use of the gain lobes but not on a vertically mounted VHF/UHF version. I think hertzian ran an EZNEC plot of this and it should show where the OCFD might be useful and where it is not.
prcguy

What would be interesting to see is an SWR plot from an antenna analyzer like the Comet CAA-500... showing the "sweet spots" of resonance across say, 50 mHz to 500 mHz of the OCFD. If I can borrow one and connect it to my OCFD in the attic, I will do so. But it may be awhile, so if someone else has a suitable plotting analyzer.... please post! Typically, an HF OCFD will have multiple resonances across a wide range of frequencies.... that is the whole point of the design. And I know that HF OCFD antennas change impedance as you go higher above the ground, and require different ratios of balun transformers if you mount them above about 50 feet. I'm assuming something similar happens with the VHF OCFD (?) At the very least, it would be interesting to run the plot with the analyzer, then move the antenna or change the feedline length.... rerun the analyzer plot and observe any changes.

 

[kandrey89]

The wires coming off a 75 to 300 ohm is not called "twinax", that's a specific type of coax with two individual and parallel center conductors inside a shield, with a whole line of coaxial type connectors to match.

The Off Center Fed Dipole is also not a balanced antenna, its very unbalanced with unequal currents at the feedpoint but it does benefit from the "balun" effect of a 4:1 transformer. The feedpoint of the OFC is not really 200 ohms or 300 ohms as reported, its all over the place on the scanner version people are building here.

When an OFC is designed for a certain frequency and used on harmonically related bands it does match ok to around 200 ohms on those specific harmonic bands but the scanner version is simply some random lengths of whatever material you make it from and can't really be compared to an off center fed "Carolina Windom" which it has some resemblance to.

I think some of the discussions of what material to make it from and how thick are almost completely irrelevant because this particular antenna is a random meaningless length on all but a few discreet frequencies. VHF low band will suffer the worst due to its length and the fact most TV transformers start dropping off in performance below 50MHz. I've measured a few TV transformers back to back with lots of loss in the VHF/UHF region and I seem to remember 3 to 5dB (half that for one) was common for some brands.

The OFC is a cute little project that many people will benefit from compared to smaller or indoor antenna, but its not a mystery or magic antenna that some are touting it to be.
prcguy

Sorry, my bad I meant Twin Lead, https://en.wikipedia.org/wiki/Twin-lead

I had read before that the OCFD had a variable impedance, while writing the above post I somehow was thinking of a balanced loop antenna, where as OCFD is not a loop, it's two broken sticks.

Makes me want to drop $3k on DSA815-TG 1.5 GHz Spectrum Analyzer | Rigol - Beyond Measure to be able to quickly measure antenna SWR. $1.5K on Spectrum Analyzer and $1.5K on tax, scope upgrade and VSWR bridge. Surprised there's not a similar solution that can record SWR as a function of frequency data into storage from the veteran companies, most of those appear to be like a quick multimeter test check tool, not a sweep and save data type tool.

 

[prcguy]

I like your description of two broken sticks, which I'll use from now on to describe the VHF/UHF OCFD.

I often use an HP8594E spectrum analyzer with tracking gen option and an Eagle 3GHZ return loss bridge for analyzing return loss. Its a great way to see the what an antenna is doing across a very wide spectrum at a glance while you make adjustments.

However my HP analyzer recently broke and will probably cost more than its worth to repair so I am shopping for a replacement while many projects are on hold.
prcguy

Sorry, my bad I meant Twin Lead, https://en.wikipedia.org/wiki/Twin-lead

I had read before that the OCFD had a variable impedance, while writing the above post I somehow was thinking of a balanced loop antenna, where as OCFD is not a loop, it's two broken sticks.

Makes me want to drop $3k on DSA815-TG 1.5 GHz Spectrum Analyzer | Rigol - Beyond Measure to be able to quickly measure antenna SWR. $1.5K on Spectrum Analyzer and $1.5K on tax, scope upgrade and VSWR bridge. Surprised there's not a similar solution that can record SWR as a function of frequency data into storage from the veteran companies, most of those appear to be like a quick multimeter test check tool, not a sweep and save data type tool.

 

[popnokick]

As I wrote earlier in this thread, I would like to use an antenna analyzer like the Comet CAA-500 to sweep my homebuilt RadioReference Antenna Wiki all-band OCFD scanner antenna from 29 mHz up to 500 mHz (the top limit of the CAA-500). And eventually I will be able to borrow one from one of the hams in our club.

Meanwhile, I realized that I have a tool that will do a portion of that range (415 to 475 mHz) - the MFJ-229 UHF Antennal Analyzer that I use with my UHF ham gear and GMRS equipment. I really want to see how “bad” (or good) the scanner OCFD is regarding SWR across the very broad range of frequencies it covers. I use mine from 29 mHz all the way up to 470 mHz. It’s mounted in my attic near a window, and connected to two scanners in the house via a PCT MA2-2P Drop Amp / Splitter. It hears anything I tune the scanners to…. and at good distances. From Northeast PA I can hear public safety, utility company, and amateur radio transmissions in this part of PA, northern NJ, and the southern tier of NY. It also does particularly well on the civil and military air bands.

All I can test with the MFJ-229 is the UHF freq range from 415 to 475 mHz, but that is in the “hit zone” of much of the discussion on this thread. I also realize that the drop amp / splitter may be affecting the SWR, so I tested both at the end of about 25 feet of coax where my upstairs scanner is normally connected, and I also went up to the attic and tested at the splitter end of the 3 foot coax to the OCFD. Here are the results:

MFJ-229 SWR readings as measured at the end of approx 25 ft of coax coming off the PCT MA2 splitter / amp -
FREQ SWR
415 mHz 1.2
420 mHz 1.4
424 mHz 1.1
425 mHz 1.0
430 mHz 1.4
437 mHz 1.2
441.5 mHz 1.4
446 mHz 1.2
450 mHz 1.1
452 mHz 1.3
455 mHz 1.3
460 mHz 1.0
465 mHz 1.2
470 mHz 1.1
475 mHz 1.2
477 mHz 1.2

MFJ-229 connected directly to the OCFD (no splitter / drop amp)
415 mHz 1.0
420 mHz 1.0
425 mHz 1.0
430 mHz 1.1
440 mHz 1.2
445 mHz 1.3
450 mHz 1.3
455 mHz 1.3
460 mHz 1.2
465 mHz 1.2
470 mHz 1.2
475 mHz 1.1

If the “look angle” of the OCFD is well above the horizon, you could have fooled me. I can always hear GMRS and ham radio repeaters in northern NJ. Some of the weaker ones in NJ are difficult to pick up when all the leaves are on the trees in the summer, but crystal clear now when the foliage is gone. All of the rest are year-round, reliable “ground wave” reception (not reflections or skip of any kind).

 

[KC4RAF]

THAT is quite an impressive antenna

you built! The swr across the frequencies you listed is great. Is your analyzer dependable? Not questioning your equipment; I had one that needed some repair, it read zero on all bands!!! lol

 

[popnokick]

My MFJ-229 is reliable.... not only with my multiple antennas, ham gear, and GMRS stuff.... but also according to a fellow Extra Class ham and former broadcast engineer who just returned it after borrowing it to finish the work on his UHF antennas. He said my meter is working well.

 

[HelixArray]

Thanks for measuring that, popnokiock!

Made a new "broken stick"...but I have a quick question, guys. In the highest point of my attic I have my copper RRwiki OCFD mounted to a 2x4 via zip ties. Can I mount my 2nd OCFD on the other side of that same 2x4?

Here is a SDR# comparison of a copper RRwiki OCFD to an electric metallic tube (EMT) conduit 3/4" OCFD, cut same length, same type transformer(RCA brand) and to the same length of RG6-quad(Lowes brand) F-type to F-type. The copper OCFD uses another brand of RG-6-U F-type to BNC.

The EMT OCFD is mounted slightly lower(5 inches) than the copper one.

I tuned to NOAA and to the 470.310 TV constant signal and had a difference of 34 SNR.dB(copper) and 27 SNR.dB(EMT)

I still haven't switch around the coax to see if it matters.

In the image below I switched both antennas out in under 2 seconds. The range looks different between the two. I still have more comparisons to do(don't know too much in this dept), but I can already tell my long range efforts are hurt ATM.

http://i.imgur.com/LkMwTB4.jpg

 

[popnokick]

You might see some interaction of the SDR stick rcvrs if you have both OCFD antennas that close to each other. Try it and see... should be easy enough to separate them if a problem develops.
Doesn't seem that copper and EMT should be that different, unless perhaps the electrical connection to the EMT is not as good as the copper. Definitely switch coax to rule that out as a possibility for the signal difference.

 

[br0adband]

34 dBm vs 27 dBm is a rather significant gain since a 3 dBm difference is effectively a doubling in power iirc so a ~7 dBm difference is quite a big one.

 

[prcguy]

I agree, there is a difference between copper and plated steel elements but it should be in the fraction of a dB range if everything else is the same. To get an honest comparison you really have to isolate just the element materials when testing so the coax, mount and precise location of the antenna should stay exactly the same. If you added another antenna nearby between tests then all bets are off on the outcome.
prcguy

You might see some interaction of the SDR stick rcvrs if you have both OCFD antennas that close to each other. Try it and see... should be easy enough to separate them if a problem develops.
Doesn't seem that copper and EMT should be that different, unless perhaps the electrical connection to the EMT is not as good as the copper. Definitely switch coax to rule that out as a possibility for the signal difference.

 

[HelixArray]

Ok, I ran out and bought me an authorized stocking stuffer per my old lady's orders. I came back with another length of copper. I have three total RRwiki OCFD's now.

Bare with me until this holiday is over. I did a first round of comparisons and attached the EMT OCFD on the other side of the copper OCFD( a fraction of an inch in height difference ) spaced by a wood 2x4 . Switched around both coax feeds. Still have roughly a 7 dB difference on the TV signal(470.310) which I'm using as it's more constant than the NOAA signals. I even switched out to a third balun on the EMT, same result. My first install of the EMT was a few inches lower of the copper one and 5ft away.

It's pretty hot in my attic this time of year believe it or not. We had severe storms last night in my area. I'll do the exact comparisons(to my ability) that you guys suggested. I appreciate your help. I know from reading older OCFD threads you 4-5 guys are pretty active posters in them. Thanks again.

 

[NDRADIONUT]

A few inches of difference in position can make quite a difference on uhf....