Possible to build antenna specific to VHF low?

[dmg1969]

I just yesterday built an OCFD antenna for my base scanner (see my other thread). I live in a ranch house and the attic does not have near enough clearance to put it up there. So, I have it propped in the corner of my living room. It is picking up VHF high and UHF just fine. My problem is that our county police, fire and EMS runs on VHF low. Dispatch comes in great, but mobiles are not. Maybe that has to do with some of the mountains around here.

What I was wondering is this. Is it possible to build a more powerful antenna geared specifically at VHF low? I am targeting the 45.10 MHz to 46.30 MHz range. Or is my problem really all about getting the one I have already built higher?

 

[zz0468]

Your poor reception of mobiles probably has more to do with an antenna propped up in the corner of the living room than anything else. Yeah, it's possible to build something better than an OCFD, but it'll suck just as badly if it's propped in the corner.

Get an antenna as high and in the clear as possible, any antenna. Then, if you still have problems, you can blame it on the surrounding mountains.

 

[popnokick]

Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 6_1 like Mac OS X) AppleWebKit/536.26 (KHTML, like Gecko) Version/6.0 Mobile/10B144 Safari/8536.25)

Dave- You most likely are looking at an outdoor, roof mounted antenna to give you a fighting chance at receiving VHF low band mobiles. BUT before you go to all the trouble of a permanent outdoor mount on the eave, gable, or roof itself, here is what you could try to see if it's going to be worth the effort. You may need some warmer weather, but up to you....You're headed for the roof.
- Would help if you had a fellow firefighter with an installed lowband mobile for testing, but not essential
- Get a camera tripod, portable bench and clamps, or some other jig that can hold your OCFD vertically on the roof or at one of the peak ends (like at apex of roof above your deck).
- Unless you want to stand on the roof during all the testing, get enough RG75 to reach from the roof at least down to the deck.... Or better, inside. This additional coax is also going to become part of the VHF low on the OCFD. Should help, but no guarantees.
- Put the OCFD on the roof temporarily.
- Connect your BC125AT and start listening to low band channels (or have your friend start running test transmissions with dispatch).
- Have the mobile drive farther away until signal is lost (or keep listening to whatever you can hear).
- This will tell you if the rooftop antenna is going to help... Or not really make a difference. THEN you can curse the mountains... Before you buy an eave mount, drill the side of the house, drive ground rod, etc.
- If there is an improvement, you can either weatherize your OCFD or get a purpose-built VHF lowband antenna on the roof.

 

[dmg1969]

Yeah, I'm going to do some testing with it this coming weekend. I was hoping to avoid the outside installation but, if I truly want to hear the further mobile units, I don't think I can avoid it. I'm going to rig up something temporary on the deck enough to get it above the roofline. If it greatly improves the VHF low reception, I'll pop for the mast and grounding supplies and somehow weatherproof it.

Dave

 

[popnokick]

Just realized I wrote RG75 in reference to coax. Meant RG-6 (75 ohm coax).

 

[nanZor]

How about a half-wave RFD coax vertical antenna scaled up a bit larger than the one shown for 6meters here:

Half-Wave Flower Pot Antenna – VK2ZOI

If you have 10 feet of vertical space, you could use 2-inch pvc, about 17 to 18 turns for the coax choke made out of solid-dielectric RG-58, and 56 inches for each element length?

2-inch pvc might be a pain to mount, so perhaps just use enough of the 2-inch pvc for the coil form, and either adapt to a smaller diameter pipe, or merely hang the coaxial antenna element exposed. Might be worth looking into for something quick, easy, and cheap.

 

[GKolo]

I would love to build one for LowBand also just to pick up fire depts across the US when the bands open up in the 33 mhz and 39 mhz band, Would this be the correct antenna ?

 

[nanZor]

There are many correct choices. Depending on your needs, this may be one of them to get a taste and have some inexpensive fun. At 36 mhz, be ready for a tall antenna for a full half-wave: about 12 feet tall minimum.

A flowerpot version for 36 mhz can be made with RG-58 by cutting the black covering and the braid from 6-feet of the coax, being careful not to cut too deeply into the hard inner dielectric that covers the center insulator. That is the top half. This could be a pita to do, so one might just want to solder an insulated 6 foot wire to the center conductor of the bottom half, but this would weaken an already fragile setup if unprotected.

Protect the slightly exposed braid at the cut with some tape, coax goo, or anything to keep moisture from wicking down into the braid.

The bottom half is merely another 6-foot length of RG-58 (no cutting). At the end of that 6-foot length of the bottom half, start winding the choke coil.

The choke coil consists of a 4-inch diameter pvc coupler. Wind 4 turns of the RG-58 closely together side by side. Run the rest to the radio. The whole thing resembles that shown in the VK2ZOI link. So the pvc form for this choke would be 4 inches diameter, and maybe 6 inches long or so.

Although fragile, you could merely hang it from an overhead support. Using a 4-inch pvc pipe for the whole length would be overkill, so some ingenuity is required - perhaps use a 1-inch pipe merely for support, and have the 4-inch diameter choke piece of pvc off to one side at the bottom. Since the antenna is already covered in a dielectric material, the pvc won't radically detune it much further. You could put it inside, or even just tie-wrap it outside for a temporary setup.

The problem here will be getting it high enough for line-of-sight local reception, however if it is just for occasional skip monitoring, then it isn't quite that critical. If you can, keep it at least 6 feet away from other nearby large metallic objects.

 

[LtDoc]

There are two things that will help with hearing those low-VHF stations, height and an antenna specifically made for that frequency range. I think your present antenna is a bit 'short' for that low-VHF, but it should at least work 'better' if it was higher. Because of the area you are in (mountains, etc.) I sort of doubt you will have -reliable- 'hearing' of those mobiles. Height will certainly increase what you will be able to hear though. Above the 'roof line' is good, but higher would be even more 'gooder'. Unfortunately, you won't hear all of them probably most of the time (I'd bet that their dispatcher doesn't hear'em all the time either).
It just depends on how much time and trouble you want to go to. A very tall tower and directional antennas would make it more certain of hearing 'everything', but that's getting a bit ridiculous, huh?
- 'Doc

 

[Boatanchor]

One of the biggest problems on low band, is the amount of RF noise produced by just about every electrical appliance in your home, the neighbors homes, powerlines and even atmospheric/solar noise etc etc etc.

Relatively strong wanted signals often get drowned out in this sea of noise.

Here's a little test..

Tune to any frequency around 45Mhz, set your mode to AM and open the mute/squelch.

How is the noise level? What is the S meter doing (Note that the Uniden S meters only register noise level when the squeclch is opened or if there is a carrier on the frequency)?

While the squelch is open, disconnect the antenna from the scanner. Notice any difference to the background noise and S meter?

The noise you are hearing (there will be some, if your antenna is more efficient than a house brick and you are not living in a cave with no electricity), is degrading and reducing your reception range of distant wanted signals. The higher this 'noise floor' is, the stronger the wanted signals have to be to overcome it and be heard.

If you have a handheld scanner, leave the squelch open, plug the rubber duck antenna in and go for a walk around your home. Hold the scanner near to your TV, Computer, Microwave oven and air conditioner etc. Almost every appliance generates noise on this band unfortunately.

To reduce the noise as much as possible and to increase the signal strength of wanted signals, you have little choice but to try and get your receive antenna as far as possible away from these noise sources.
Vertical seperation is much more effective than horizontal seperation at attenuating unwanted signals/noise, so get your antenna up as high as possible above your roof. Having a higher antenna also increases the Line of Sight range of your antenna, increasing the range to distant wanted signals. Kills two birds with one stone really

 

[dmg1969]

There are two things that will help with hearing those low-VHF stations, height and an antenna specifically made for that frequency range. I think your present antenna is a bit 'short' for that low-VHF, but it should at least work 'better' if it was higher. Because of the area you are in (mountains, etc.) I sort of doubt you will have -reliable- 'hearing' of those mobiles. Height will certainly increase what you will be able to hear though. Above the 'roof line' is good, but higher would be even more 'gooder'. Unfortunately, you won't hear all of them probably most of the time (I'd bet that their dispatcher doesn't hear'em all the time either).
It just depends on how much time and trouble you want to go to. A very tall tower and directional antennas would make it more certain of hearing 'everything', but that's getting a bit ridiculous, huh?
- 'Doc

LOL! If space and money were no object, I would have a HUGE tower. I haven't had much time to look into it lately due to my mother being in the hospital and then rehab for a back injury. For now, the dipole is doing a fair job on VHF low.

 

[LIScanner101]

I recall testing a 108" metal whip CB antenna that I cut-down to exactly 1/4W length at 46.12MHz (5.33', or 5' 3.9"). I mounted it onto a neat NMO base that I was then able to screw-onto my RV that has an NMO-to-N connector mounted on the huge metal roof (the NMO outside obviously, the N female facing down inside the ceiling in a cabinet).

Using a 10' long LMR-400 "pigtail" I then connected my scanner and listened...

FIRST, I tried an old Antenna Specialist MON-51 and an Austin Spectra. As expected, the A/S kicked butt on 46.12MHz, the Spectra did pretty well.

THEN, I tried the 1/4 wave whip. HORRIBLE!!! The noise and static was through the roof (no pun intended). I was SURE this was going to be a killer VHF-lo antenna, but it turned out to be a piece of crap. It was unlistenable. Even strong signals sounded awful.

I think I just chopped it up & tossed it.

I guess it was just a "noise magnet"!!

 

[nanZor]

Re the NMO - I've been burned by mixing and matching NMO mount products where the center flat finger of the NMO did not mate properly to the center conductor. Either the original rubber O-ring was too tall, or the borrowed nmo antenna had the internal finger smashed upwards, and was only making a capacitive coupling. Not saying that was the problem in your case, but it certainly was in mine on a project years ago.

One way to get down to about 35 mhz is to couple to amateur mobile masts together:

1) The Hustler MO-2 is 54" long and by itself is resonant around 52 mhz.
2) The Hustler MO-4 is 22" long, and by itself is resonant around 128 mhz. (makes a great hardcore airband quarter wave)

Couple both of these together with a 3/8" long nut from the typical mirror-mount, use the right groundplane, along with rf-choking at the feedpoint, and it will be guaranteed to rip the sheetmetal off your vehicle unless guyed.

You could mount this in a fixed situation with a few elevated radials, or a large ground plane. 35 mhz +/- will never sound the same - as long as the noise issues are dealt with.

 

[LIScanner101]

Re the NMO - I've been burned by mixing and matching NMO mount products where the center flat finger of the NMO did not mate properly to the center conductor. Either the original rubber O-ring was too tall, or the borrowed nmo antenna had the internal finger smashed upwards, and was only making a capacitive coupling. Not saying that was the problem in your case, but it certainly was in mine on a project years ago.

It was definitely not in this case. I used a caliper to check all mating parts of the NMO mount “button” and the tab on the antenna NMO part – it was a perfect match. I also checked for continuity with a DMM after I attached the antenna. Lastly, since this mount is smack-dab in the middle of my RV (which has a huge aluminum roof) it has an excellent groundplane.

 

[abbadabbado]

vhf low

in the mountains of eastern ky southwestern va i use a plain old cb ground plane antenna 50 feet of cable i receive low band signals from 100 miles away i done this by accident lucky me

 

[misterpaul71]

Hi,

I use a rectangular loop cut to 33.7 mhz here in Ireland and I can hear US low VHF traffic most days at the moment. If you are interested in hearing "skip" signals then this might be an idea.

The loop is 59 and 2/3 inches top and bottom and 119 and 1/4 inches on the sides.

This is supported on a tall fishing pole the top and bottom wires are taped to dowel rods and zip tied to the pole.

Coax is fed in to the middle of the bottom side (the feed point because of the rectangular shape is close to 50 ohms)

You can add a coax balun at the feedpoint if you like - i think I have a 6 turn coil of coax on a 4 inch former.

Its good on receive from say 27 mhz up to say 37. When propagation conditions are good you can extend those numbers upward.

fed at the bottom middle this antenna is horizontally polarised - which will knock back some local noise - most of which is vertically polarised. For local vertical reception the antenna could be fed in the middle of one of the sides.


Paul in Ireland.

 

[reconrider8]

Pic?

 

[misterpaul71]

Low VHF loop

I just tried out the loop against an OCFD today, the OCFD was at similar height and showing good SWR across 31- 34 mhz. The loop beat it out easily.

Heres a pic.

The antenna on the left is a 1/4 wave cut to the bottom end of 10 metres.

 

[popnokick]

Wirelessly posted (Mozilla/5.0 (iPhone; CPU iPhone OS 7_0_4 like Mac OS X) AppleWebKit/537.51.1 (KHTML, like Gecko) Version/7.0 Mobile/11B554a Safari/9537.53)

And fits in the attic... Just like OCFD..... NOT!

 

[misterpaul71]

low VHF Loop

Actually if you put it on its side you might fit it into an attic. An OCFD if its going to be any use at low vhf is gonna end up in excess of 13 feet anyway.