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  #1  
Old 01-22-2019, 11:45 AM
maniago's Avatar
maniago maniago is offline
 
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Default TV antenna for VOR

Can anyone tell me why using an old set of rabbit ears (set to 26" or so) wont work for a VOR/LOC/GS antenna (under canopy install)? Ham guys tell me that using the 75-300Ohm balun is fine for 50ohm coax work (and it has the freq range)....
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  #2  
Old 01-22-2019, 12:06 PM
rv7charlie rv7charlie is offline
 
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Strips of copper tape around the back of a M-II canopy work well, and 'cat whisker' VOR antennas are basically the same thing, so why not? I don't think the copper tape antenna even uses a balun, but I can't find my copy of the plans at the moment.

Of course, leaving the rabbit ears out in the breeze might present some issues...

Charlie
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  #3  
Old 01-22-2019, 12:31 PM
Canadian_JOY Canadian_JOY is offline
 
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Location: Ontario, Canada
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Mani - using the 75ohm balun will produce some signal loss. Not bad enough to generally be a problem, but you will lose some range on the VOR. Where it will get more problematic is if you use a splitter downstream to split off to another NAV radio, or even splitting off the Glideslope signal. Some splitters are quite impedance-sensitive. I know you're running a GNS480 which has an internal splitter - I don't know how impedance-sensitive it might be.

On the other hand, there's no reason why you couldn't build a 50 ohm balun - it's not rocket science.

Then again, copper foils with a balun at their near-junction point would be more elegant and likely would cost you only a very few dollars more than your TV antenna setup.
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  #4  
Old 01-22-2019, 12:47 PM
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maniago maniago is offline
 
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Charlie - its under the canopy, so no breeze other than typical drafty-ness.
Also, theres a copper antenna design in the plans? Mustve missed that (ignored all the electrical and panel pages).

Canadian, yeah I have a splitter, 480 and SL30 feed from it. So I might be pushing my luck, but the TV antenna was waiting for the basement clean out anyway so its no money. Any $300 Rami antenna I buy is going to get the same splitter treatment tho.....

I gotta figure that some kind of balun is better than no balun, as most DIY antennas are running raw. Was thinking of getting a cheap VSWR meter and putting it inline and tuning the antenna length - cant get much easier with telescoping legs Id think.....
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Last edited by maniago : 01-22-2019 at 12:50 PM.
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  #5  
Old 01-22-2019, 01:38 PM
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Carl Froehlich Carl Froehlich is offline
 
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Please don't put this in your airplane.

Some thoughts:
- A 'V' or a dipole antenna is already close to 50 ohm impedance. No balun required. Some old school thought is a 1 to 1 balun to transform from balanced to unbalance feed but years of data shows the gain from this is so small it is in the grass.

- Any dipole or V can be made from a couple of pieces of wire, one soldered to the coax center conductor, one to the coax shield. The wire lengths will be 25" or so. You can run the wires out from the center as desired, but recommend they run perpendicular to the fuselage axis. If you want it to conform to the canopy recommend the center feed point up high and the legs run down from it. The first third of the antenna does 90% of the work (the rest is there to bring it to resonance and such).

- Any antenna mounted in the cockpit, or under the cowl will pick up a lot of RFI. In the cockpit it will get it from all the glass panel switching power supplies. Under the cowl it will get ignition noise from being close to the ignition wires, mags and plugs. For this reason I recommend you seek a better location. Is there reason not to put it in the wingtip?

Carl
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  #6  
Old 01-22-2019, 02:32 PM
rv7charlie rv7charlie is offline
 
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Not in the plans, but it was a common technique in both the M-II and the T-18 back in the 80s & in the early 90s, when I had a T-18, and M-II project for a while. The center point is at the rear of the canopy, and the legs extend around/forward a few inches above the canopy skirt. I've got the paper drawings for it around the house somewhere. If you want to play with the idea, I've got several rolls of the foil lying around, or you can order some from Spruce. Here's a link to Jim Weir's antenna book, describing how to build a foil antenna.
https://www.google.com/url?sa=t&source=web&rct=j&url=http://www.rstengineering.com/rst/products/plasticplaneantenna/plasticplaneantenna_files/2802%2520Manual-s.pdf&ved=2ahUKEwiQyfWAmYLgAhUEOq0KHXHqAEMQFjAAegQ IBBAB&usg=AOvVaw2YL3UUMehtH-GUUM0hFaW_

If the paper copy of the M-II/T-18 install turns up, I'll let you know. You might try asking over on the M-II forum, if it's still active. If any of the old guard is still around over there, they might have a copy.

Charlie

oh, edit: don't get too wrapped around the ferrite baluns in the article. I doubt you could tell the difference with a receive-only antenna.

Last edited by rv7charlie : 01-25-2019 at 07:09 AM.
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  #7  
Old 01-25-2019, 03:48 AM
Rainier Lamers Rainier Lamers is offline
 
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While experimenting with various antenna types during the development of the MGL N16 navigation radio I can conclude that pretty much all of the solutions presented in various pieces of literature work quite OK.
We are only receiving and this makes it somewhat less of an issue.

The main differences between solutions are overall sensitivity and directional characteristics. Sensitivity does not appear to vary too greatly as long as the structure resonates at the desired frequency but the directional characteristics can be a bummer so this needs a bit of attention.

A potential problem we found relates to the 50 ohm coax cable and it seems to make not much difference what type you use. For all types of dipols or related structures, with or without baluns we found the coax cable shield becomes live and a part of the antenna - and more than just a small part. If the antenna presents a perfect match to the cable the cable shield should be dead. This seems difficult to achieve for these antennas.
In most cases this matters little - but it can become a problem if the antenna cable happens to "receive" interference from on board electrical sources. As the cable is typically routed close to potential sources of such interference in the cockpit it can become an issue.

Practically this can express itself in unsteady VOR or localizer output on a frequency subject to interference.
Glide slope, while theoretically similar - we found to be somewhat more robust - perhaps due to typically reduced power of interfering harmonics at the higher frequencies used for GS.

Interestingly one of the best antennas we tried was a simple piece of wire perhaps 2-2.5ft long danging directly from the antenna connector of the receiver in "messy" fashion (this in a "plastic" aircraft). This was just an initial attempt to check that all was working before trying real antennas - we found most alternative antenna solutions did not perform better and often worse in some or other way (most of the tests where "rough" so your mileage can vary - it was just done to get a feel for this).

The traditional VOR/LOC/GS antennas we all know so well (towel bar types, or tail mounted V types) worked well. Still some directional issues but well defined such that the maximum sensitivities are where they are the most useful. Perhaps not the most sightly and a bit of drag perhaps but personally those are my choices.

What we found helps with a live antenna cable shield - route the cable tightly against your metal fuselage without any physical space for as much distance as possible - then when it needs to get to the nav radio route it with as short as possible path through the air or if possible against metal supports as much as possible, all the while avoiding proximity to other electrical cables and potential interference sources like EFIS systems, any form of LCD display or other digital electronics or switched power supplies.

Rainier
CEO MGL Avionics
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  #8  
Old 01-29-2019, 01:41 PM
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maniago maniago is offline
 
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Quote:
Originally Posted by Rainier Lamers View Post
While experimenting with various antenna types during the development of the MGL N16 navigation radio I can conclude that pretty much all of the solutions presented in various pieces of literature work quite OK.
We are only receiving and this makes it somewhat less of an issue.

Rainier
CEO MGL Avionics
Thanks Rainier - your whole write up was very helpful, esp the notes on live shielding vs balun use.
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  #9  
Old 01-30-2019, 09:44 AM
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maniago maniago is offline
 
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Quote:
Originally Posted by Rainier Lamers View Post
What we found helps with a live antenna cable shield - route the cable tightly against your metal fuselage without any physical space for as much distance as possible - then when it needs to get to the nav radio route it with as short as possible path through the air or if possible against metal supports as much as possible, all the while avoiding proximity to other electrical cables and potential interference sources like EFIS systems, any form of LCD display or other digital electronics or switched power supplies.

Rainier
CEO MGL Avionics
Rainier,
What about double shielded RG400 coax, such as this.....https://fieldcomponents.com/RG-400-M...CABEgKa8_D_BwE

...at $2/ft, its cheaper than regular RG400 from Spruce etal....tie the outer shield to the radio case, leave the other end floating, and no live shield to act as an antenna, no?
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  #10  
Old 01-31-2019, 03:31 AM
Rainier Lamers Rainier Lamers is offline
 
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Quote:
Originally Posted by maniago View Post
Rainier,
What about double shielded RG400 coax, such as this.....https://fieldcomponents.com/RG-400-M...CABEgKa8_D_BwE

...at $2/ft, its cheaper than regular RG400 from Spruce etal....tie the outer shield to the radio case, leave the other end floating, and no live shield to act as an antenna, no?
Sadly our experiences with RG400 show that it is not a plaster to fix antenna problems. It is good cable with very low loss as long as it is used correctly. To be honest, provided you have a well matched antenna, there is not much difference to be had between a good quality RG58 and RG400 at VHF frequencies for our use.

It is often thought (and that included me) that the extra shield somehow isolates the inner cable from interference etc. But that does not happen this way (sadly).

Essentially the inner core and the shield forms a transmission line. If all if perfectly done the signal energy travels as a wave via the center conductor and the capacitively and inductively matched shield assists in the wave traveling without radiating anywhere. The result is that all the energy arrives at the destination.
If the shield is not correctly terminated at any of the two ends (the antenna side often being the problem area) the shield does not work correctly. More to the point it becomes "live" - part of the signal traveling in the inner core is now free to couple to the shield which also gets any reflections from the antenna - just to make things worse. Of course since it has some length it forms an antenna that will resonate at a couple of frequencies based on its length but also on how it is routed (you can create shorter antenna stubs in all sorts of ways here) - the result is usually a very complex antenna (not a very efficient antenna most of the time). All of this works for receive as well - the shield receives RF and couples it to the inner core.

A live shield is what often results in radio interference - other instruments going crazy when you transmit and easily receiving interference from strobes to anything digital (in particular if it has a big LCD display).

Using an SWR meter is often used to check the quality of your installation and this is a good way - don't be fooled by statements like "2.0 is good enough". That's fine for radio amateurs 30 meter rig but not for your installation. You should really try and get it below 1.4 if you have any interference.
Now, just a word of warning - I have now come across installations using dipols with typically three ferrites mounted at the base of the antenna over the shield as some sort of balun. These arrangements tend give a very good SWR but actually do not work !!! Instead the reflected energy of the mismatched antenna is simply prevented from returning by the ferrites giving the impression that all is well. Instead, from an RF point of view, the shield is disconnected at the antenna. A quick check with a field strength meter shows that effectively the entire antenna cable is your antenna (the shield is really very live) and the half of the dipol that is attached to the core does some radiation as well - the other half does nothing much.
These antennas are becoming popular in some aircraft as you can hide them but in my experience are a really bad idea.

Back to using antennas like this for NAV (dipols with proper baluns or matching transformers or shaped to present a good match by other means) - generally they work OK. RX is somewhat less critical than TX but you still need to be aware of interference which can affect your NAV radio.
You will likely have your NAV radio plugged into an intercom system.
Switch all of your systems on (except the engine) and scan through all channels starting at 108 Mhz while listening to the background noise. Make sure none of the channels gives you interference - this should be audible. If you find a channel - switch off equipment one by one. If all is off (except the radio and intercom and nothing changes - the interference likely comes from external to your aircraft (Florescent tubes in your hangar perhaps ?).
If you do not find a single channel with interference you cannot live with - you're antenna should be good to go.
You can do a final check with engine running - just to make sure your ignition system is not a potential interference source.

Rainier
CEO MGL Avionics
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