First post! Antenna questions

nikki62 · #1 (**permalink**) 10-06-2012, 9:07 PM

Hi everyone,

I'm mainly interested in airband monitoring with a little UHF (emergency services etc) on the side. I have a Uniden 996 and a SBS-1 ADS-B unit as well.

Quick background - i live in a rental townhouse with a flat metal roof. In the past, i've secretly stuck up two cheap magmount whips - one dedicated to airband and the other a wideband whip. Also ADS-B magmount (pic attached). They worked ok but i think i can do better. As you can see from pic, the mag mounts did rust away after about two years so i pulled them all down.

I'm now keen to put up some new antennas however i'm not sure landlord will let me mount a discone etc. I've attached a second pic of my metal roof to see what ideas anyone may have for antennas. I still think magmount whips may be the way to go. I noticed that Laird seem to make weatherproof mag mounts. I'm also wondering what the best mobile antenna would be for airband monitoring.

Any thoughts would be greatly appreciated. Thanks.

mmckenna · #2 (**permalink**) 10-06-2012, 9:11 PM

Good mag mounts should not rust like that.

If you can't permanently mount antennas, it looks like a mag mount, or some sort of tripod mount might be your best bet. With the metal roof, trying to do something inside is likely going to have poor results.

I'd suggest staying away from the cheap ones. I've had really good luck with the Larsen brand products, although I don't use mag mounts except for testing or emergencies.

scottyhetzel · #3 (**permalink**) 10-06-2012, 9:13 PM

Hi Nikki,

It would be better if you mount the magnetic mounts to a flat piece of metal...the antenna work better on a ground plane.. The air band is AM so it will increase the antenna gain. Just try to mount it on the highest point like that tank. Also space the antenna apart like 6 feet. They loss signal when they are with in their individual wavelength.

prcguy · #4 (**permalink**) 10-06-2012, 11:01 PM

Air band is AM so it will increase antenna gain? Can you explain that a little more?
prcguy

Quote:

Originally Posted by scottyhetzel

Hi Nikki,

It would be better if you mount the magnetic mounts to a flat piece of metal...the antenna work better on a ground plane.. The air band is AM so it will increase the antenna gain. Just try to mount it on the highest point like that tank. Also space the antenna apart like 6 feet. They loss signal when they are with in their individual wavelength.

gewecke · #5 (**permalink**) 10-07-2012, 2:00 AM

Quote:

Originally Posted by prcguy

Air band is AM so it will increase antenna gain? Can you explain that a little more?
prcguy

* delete*

gewecke · #6 (**permalink**) 10-07-2012, 2:06 AM

Quote:

Originally Posted by scottyhetzel

Hi Nikki,

It would be better if you mount the magnetic mounts to a flat piece of metal...the antenna work better on a ground plane.. The air band is AM so it will increase the antenna gain. Just try to mount it on the highest point like that tank. Also space the antenna apart like 6 feet. They loss signal when they are with in their individual wavelength.

Also mounting mag antennas on the tank in the photo horizontally may work well too,since some aircraft signals are transmitted horizontally. That roof if steel has a lot of antenna choices!

73,
n9zas

popnokick · #7 (**permalink**) 10-07-2012, 8:46 AM

Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 6_0 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/6.0 Mobile/10A403 Safari/8536.25)

Since the following appear to be (or may be) true based on your pics & descriptions:
- You are practiced in stealth installs (mag mount antennas)
- You or someone has quite a few cables running around atop the metal roof
- There is at least one vent pipe already on the roof
- Mounting on the tank in the pic may draw attention (but would probably work well)
Consider this: Ventenna
http://www.ventenna.com/

scottyhetzel · #8 (**permalink**) 10-07-2012, 3:17 PM

AM bounces of the ion sphere.... AM antenna need a big ground plane.... FM is direct line of site. Having the ground plane will increse the signal a lot better. A good magnet mount on a large flat piece of steel larger than 24 inches square would be ideal.

LtDoc · #9 (**permalink**) 10-07-2012, 4:30 PM

Propagation (how the signal travels) has nothing to do with the mode of use (AM,FM) but with the frequency of use. At VHF/UHF AM is as line-of-sight as FM or sideband, or CW, whatever. At HF all modes are subject to 'bouncing' off of the various ionospheric layers.
Mag-mount antennas use capacitive coupling with the metal they are attached to and that provides the 'other-half' of the antenna instead of a metal to metal contact. The surface area of the magnet acts as one plate, the metal it's attached to acts as the second plate of a capacitor. That's how all of them work. If you provide enough metal to act as the antenna's 'other-half' it tends to work. If there isn't enough metal in that 'other-half' then it doesn't work too well. It's better to have too much than too little. Magnetism plays no part in how well the antenna works, it's only use is to hold the antenna in place.
Placing an antenna as high as possible, away from any obstructions, is a very nice thing to do. There are limits to that, so work within your limits.
- 'Doc

hertzian · #10 (**permalink**) 10-09-2012, 12:58 AM

Quote:

Originally Posted by nikki62

I've attached a second pic of my metal roof to see what ideas anyone may have for antennas. I still think magmount whips may be the way to go. I noticed that Laird seem to make weatherproof mag mounts. I'm also wondering what the best mobile antenna would be for airband monitoring.

You may want to take a look at DPD Productions:
DPD Productions - VHF Air/Aviation Band, ACARS & ADS-B (SBS / RadarBox) Antennas for Commercial & Hobbyist Applications

While you may not be able to get away with the Air-Omni X model from a visual standpoint, you could certainly replace the existing magmounts with their custom tuned ones, or possibly get away with the 1/2 wave outdoor model on a ventpipe.

nikki62 · #11 (**permalink**) 10-10-2012, 1:12 AM

Thanks so much for all the fantastic replys - much to think about. I'm thinking that the mag mounts on the water tank may be the best option. Attaching an antenna to one of the roof vents is very, very tempting however i just know that if something happened to the roof that wasn't even connected to the vent/antenna, my landlord would hold me responsible anyway - he ain't a very nice person. Mind you, he did eventually find out about the previous mag mounts i had on the roof and didn't seem that concerned!

I have a really silly question to ask - the water tank that is visible in the photo is actually a solar hot water system. If i did mount antennas on the tank, would the constant heat from the tank damage a mag mount over a period of time?

Also wondering if anyone can advise on the best sort of mag mount antenna for airband?

Thanks again.

mmckenna · #12 (**permalink**) 10-10-2012, 5:44 PM

Heat can soften the cable jacket and cause problems. Finding a way to route the cable away from anything that gets hot is a good idea.
And, just in case, since it's out in the sunlight, make sure anything you use will stand up to UV exposure. Black coaxial cable is fine, but some of the fancy clear ones are not designed for use outdoors. Anything you use to secure cabling needs to be UV resistant also. Clear or "natural" colored tie wraps will break down quickly and crumble after being exposed to the sun. Use the black ones outside, they have UV inhibitors in them that will prevent this.

LtDoc · #13 (**permalink**) 10-10-2012, 9:14 PM

I guess it would just depend on how hot that tank gets. If you can touch it with no problems then I would think that mag-mount would do just fine. You might support the coax with something so it doesn't get really hot, a piece of wood sounds like it would work for that.
If that's your mag-mount antenna at the far side of that roof, it should 'work' right there. On top of that tank would probably get it above nearby obstructions, so that'd work too. Anything around that's higher than that tank? Can it still be reached with the cable you have now? A little imagination isn't going to hurt anything (until you get caught?

).
- 'Doc

prcguy · #14 (**permalink**) 10-10-2012, 9:28 PM

Never mind. If I ask scotty to explain something again would someone please kick me.
prcguy

Quote:

Originally Posted by scottyhetzel

AM bounces of the ion sphere.... AM antenna need a big ground plane.... FM is direct line of site. Having the ground plane will increse the signal a lot better. A good magnet mount on a large flat piece of steel larger than 24 inches square would be ideal.

wyomingmedic · #15 (**permalink**) 10-10-2012, 9:58 PM

Quote:

Originally Posted by scottyhetzel

AM bounces of the ion sphere.... AM antenna need a big ground plane.... FM is direct line of site. Having the ground plane will increse the signal a lot better. A good magnet mount on a large flat piece of steel larger than 24 inches square would be ideal.

Radiation itself bounces off of the ionosphere. It has to do with the wavelength of the radio waves and what layer of the ionosphere those waves are hitting. Along those lines, radio waves can also pass completely through the ionosphere if they are the correct frequency. Look up an "MUF map" which will provide a pictorial representation of the maximum usable frequency over a certain path.

Of course, this varies by time of day/season/sun activity/etc. You can also see the daily data that the HAARP project publishes.

Mode has absolutely nothing at all to do with propagation. Now, some modes are more efficient than others. AM is about as inefficient as possible. FM isn't far behind. When ham operators are communicating over a certain path (California to New York for instance), they may be able to talk using SSB. If they switch to AM, they may no longer be able to hear each other. If they switched to CW, it may be booming strong. All of this on the same frequency. It has to do with bandwidth and the Signal to Noise ratio.

But using the broad strokes of a brush to say that AM is for long distance and FM is just line of sight, is completely wrong. Frequency frequency frequency, then toss is mode efficiency.

WM

hertzian · #16 (**permalink**) 10-11-2012, 2:27 AM

Quote:

Originally Posted by nikki62

Also wondering if anyone can advise on the best sort of mag mount antenna for airband?

The DPD productions link above has a full line of airband antennas - the magmounts are near the middle of the page with whips cut specifically for vhf airband.

In a pinch, you could use an amateur 2m antenna, but why not do it right and have the whip cut at approximately 22 inches.

nikki62 · #17 (**permalink**) 10-11-2012, 5:53 PM

Thanks Hertzian. Yep, i actually emailed Dave from DPD a little while ago to see if the OmniX could be mounted on a mag mount. Dave was very helpful so i will probably order one of his antennas when i sort out what the best option is.

hertzian · #18 (**permalink**) 10-12-2012, 3:05 AM

I got a feeling that you won't be wanting to mount the omni-x on a magmount even if you could somehow.

The other issue is that it in addition to vhf air, it also is a vertical dipole for UHF milair. Unless you have milair capabilities, that antenna would be a waste.

The air-vertical on a ventpipe is only for vhf air, and would be easier to mount as long as you had some clearance from other metallic structures. Again, no magmount here either. Plus it has an N-connector, which you'd have to adapt.

I think the quickest and easiest improvement would be the quarter-wave magmount whip for now. If you also do milair, that's another story.