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The Perfect HD Antenna!

6698 Views 23 Replies 10 Participants Last post by N7BFD, Dec 18, 2006

N7BFD

Discussion Starter · May 8, 2006

I just thought it was worth a *slightly* misleading teaser headline to encourage people to experiment. For indeed I *have* found the perfect HD antenna...for me!

I decided to get the JVC KD-HDR1 car unit for use at home in part because it had external antenna input (of course) and an infrared remote. Decent price, too.

I soldered up a little motorola-to-F connector pigtail and attached a dipole I had (that had a balun with F connector). I could get all the local HD stations *except* the local NPR affiliate's HD channels. The reason I bought the radio! I have a nice hill between me and their transmitter, but I get enough signal to get nice, steady stereo on an old Pioneer tuner hooked up to an old Bic "Beambox" antenna. I'm only 4 miles away! Humph!

So I hooked up the beambox to the JVC, then an old ratshack powered antenna, then an omnidirectional Antennacraft FMSS, then a directional ratshack 15-2163. Nada.

So I sulked for a couple of days but then I went to the hardware store and Ratshack. I got:

- an 8-foot length of 1/2" copper tubing

- a 1/2" PVC "T" pipe coupler

- 2 1" long #10 machine screws

- 2 #10 wingnuts

- 4 #10 washers

- 1 300 Ohm to 75 Ohm antenna balun with F connector

Total cost about $15

So I cut 2 3-foot lengths of the copper pipe, stuck 'em in the coupler, and marked a spot to drill through each piece of pipe about 3/4" from the edge of the coupler. I drilled, put the pipes back in the coupler, put the screws through with washers and then fastened the two tails of the $4 balun with the wing nuts. Hooked it up to the JVC with a 6-foot length of coax.

BANG! All three NPR affiliate HD channels.

So I checked the rest of the channels and all was pretty good. Oddly, I lost HD on another station -- one that had been the strongest. So I took a chance and shortened the arms of the dipole tubes down to 2' 8", closer to the theoretical optimum for a dipole aimed at the middle of the FM band. Tada! I was then able to get that channel back by rotating the antenna.

So, sometimes it may be possible to fix a problem by just throwing more copper at it.

I will now proceed to paint this work of art black and admire it.

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1 - 20 of 24 Posts

Ericglo

· #2 · May 10, 2006

Quote:

Originally Posted by N7BFD
So I checked the rest of the channels and all was pretty good. Oddly, I lost HD on another station -- one that had been the strongest. So I took a chance and shortened the arms of the dipole tubes down to 2' 8", closer to the theoretical optimum for a dipole aimed at the middle of the FM band. Tada! I was then able to get that channel back by rotating the antenna.

Umm, I am not sure where you got this. The 1/2 length for a dipole should be around 60 inches as the wave is around 120 inches (300 divided by 100hz gives 3 meters). It is great that you decided to DIY and I really like your antenna. Give us some pics.

Ericglo

N7BFD

Discussion Starter · #3 · May 11, 2006

Quote:

Umm, I am not sure where you got this. The 1/2 length for a dipole should be around 60 inches as the wave is around 120 inches (300 divided by 100hz gives 3 meters). It is great that you decided to DIY and I really like your antenna. Give us some pics.

Ericglo

I should have qualified "theoretical". I looked at several different approaches including folded dipoles and asymmetrical-unbalanced dipoles, but I went with 1/2 wave dipole like that seen at (URL I can't post yet!) with each arm of the dipole approximately 1/4 wave.

I am getting worse and worse at math and I saw several different formulae, so that's one reason I decided to "go long" and then trim. Now, if I had room in my condo for a 9-10 foot long dipole, I would give that a go. Hmmm, I could hang it in the hallway.

The only relevant part of this construction project is in the attached pic. Hmmm. Not attached. Not until I hit 5 posts!! I am looking for some fancier couplers. West Marine sells sleek black fittings for bimini tops that could work, but I would have to rebuild with 3/4" copper.

Any theory aside, I just completely psyched that it works.

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N7BFD

Discussion Starter · #4 · May 11, 2006

I am not a number! I am a human being!!

N7BFD

Discussion Starter · #5 · May 11, 2006

North Country Radio dipole URL

Antenna picture attached: what could be uglier/cheaper?

Ericglo

· #6 · May 11, 2006

Man, I am an idiot. Each arm is 32 inches. I was thinking the whole thing was 32 inches. You may want to check out this site . I researched the hell out of antennas awhile back. I started a thread about it to hopefully help others.

Ericglo

Bob Smith

· #7 · May 13, 2006

Quote:

Originally Posted by Ericglo
Umm, I am not sure where you got this. The 1/2 length for a dipole should be around 60 inches as the wave is around 120 inches (300 divided by 100hz gives 3 meters). It is great that you decided to DIY and I really like your antenna. Give us some pics.

Ericglo

Actually, you need to be about 5% shorter for a half wave dipole due to the "end effect" (the ends are capacitive, and you need to make it shorter to make up for this)

The standard formula is 468/Frequency in MHz for length in feet. The geometric center of the FM band is SQRT(88*108)=97.5MHz, therefore the length should be about 57.6 Inches.

Not that all this matters much when it it indoors, it should be in the clear for at least a few wavelengths for it to work properly.

Bob

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Ericglo

· #8 · May 13, 2006

Bob,

You are correct. It is talked about in the first link I have. He actually put the adjusted lengths in his table. I just was to lazy to bring it up.

Ericglo

goobenet

· #9 · May 13, 2006

HD Radio is In-Band, On-Channel, you just need the same antenna used for FM/AM reception. It's not like it's on a seperate band or anything.

I've got a preamp on my dipole from ratshack. Works just fine.

N7BFD

Discussion Starter · #10 · May 14, 2006

Quote:

Originally Posted by goobenet
HD Radio is In-Band, On-Channel, you just need the same antenna used for FM/AM reception. It's not like it's on a seperate band or anything.

Not on a separate band? Why, that's right! I read that somewhere.

Quote:

I've got a preamp on my dipole from ratshack. Works just fine.

I am glad. If it did not work just fine, I would encourage you to experiment

goobenet

· #11 · May 19, 2006

Me too, i'd hate to have to go get a big 200" 90-element yagi just to pull in 20 minutes of advertising an hour. =)

Although i have built my own yagi's, they never seem to work as well as i'd hope... perhaps i should stop cutting the pipe with a hydraulic wood splitter?

scowl

· #12 · May 19, 2006

Quote:

Originally Posted by goobenet
Me too, i'd hate to have to go get a big 200" 90-element yagi just to pull in 20 minutes of advertising an hour. =)

Three years ago I put up a Yagi to get 20 minutes of advertising an hour from my local HDTV stations. As a one-time cost, it was a fantastic deal. My stupid cable costs more every month than that antenna!

But remember that the HD2 and noncommercial stations don't have advertising (but some sure like to regularly remind you how many sponsors they have).

Ericglo

· #13 · May 20, 2006

Quote:

Originally Posted by scowl
Three years ago I put up a Yagi to get 20 minutes of advertising an hour from my local HDTV stations. As a one-time cost, it was a fantastic deal. My stupid cable costs more every month than that antenna!

Yeah, no kidding. I have about ten to fifteen dollars in my Yagi and it was worth every penny. It is funny how people dumped their antennas in the eightees for cable. Now, it is better to have an antenna than receive the cable feed.

Ericglo

gripl

· #14 · Jun 28, 2006

N7BFD,

Thanks for sharing this. I'd like to try something similar, as I'm having trouble getting a nearby HD station (but can receive some more distant ones). A couple of questions:

1. I thought that folded dipoles needed the 300ohm-75ohm transformer, but that simple dipoles such as this have impedence of about 75ohms and so can go directly into your 75ohm input without the need for a transformer.

2. Anyone know why the copper tubing makes a difference (compared to wire)?

k2koq

· #15 · Jun 28, 2006

Quote:

Originally Posted by gripl
N7BFD,

2. Anyone know why the copper tubing makes a difference (compared to wire)?

A dipole is cut to a specific length for a given frequency. Using a fat pipe as apposed to a thin wire gives it a wider optimum frequency range.

afw7962

· #16 · Jul 6, 2006

Good job on "The Perfect Antenna"! I also have the same JVC HDR1 radio in my car. I'm in the Akron Ohio radio market not too far from Cleveland but a little too far for me to firmly lock onto WJR 760 AM's IBOC. I wished that someone with access to that radio, could work on a dual-band preamp to give the AM side more usable sensitivity. The FM selectivity of this set is pretty good, but the AM usable sensitivity is horrible compared to the stock Delco unit I took out of my Chevy last March. I personally think that the chip tuner they use in there, may be subceptable to in-band strong signal AGC Ducking. I say this because on my way to work, I pass by a directional AM station at 1350khz that "pulls" down a News-Talker (5kW) located on 640 kHz. Hopefully, someone can talk me into trying to find a preamp that won't hurt the radio's HD AM reception. I notice that I have to switch off my FM preamp to get every last drop of AM HD reception capability. I believe that the JVC KD HDR1 set could be at least 10dB less sensitive on the AM side than the stock Delco I pulled out A though for the engineer types out there, I'll bet that the preamp will have to be linear in nature to protect the digital signal integrity since ANYTHING that can obscure AM IBOC WILL ubscure AM IBOC. WJR does it right: decent stereo on their ID's and bumpers. I want the luxury of not having to travel to Cleveland to hear their IBOC clear enough to get their stereo rather than clear mono HD audio.

Thanks for any replys on this subject encouraging or discouraging the use of an inline antenna preamp for mobile HD radio signal enhancement.

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Paul Clark

· #17 · Oct 30, 2006

Most FM stations today use right-circular polarization, but some use left. For best reception the signals of interest from a given direction should all have the same circularity sense, or they should be linearly polarized.

Paul Clark

· #18 · Oct 30, 2006

Few stations today use pure horizontal or vertical polarization. Most that do are low-power translator or booster stations that extend a station's service area, or stations at the low end of the FM band that use vertical polarization to avoid interfering with a horizontally polarized TV channel 6 sound carrier at 87.75 MHz.

Paul Clark

· #19 · Oct 30, 2006

Maximum signal strength between stations occurs when both stations are using identical polarization.

Paul Clark

· #20 · Oct 30, 2006

On line-of-sight (LOS) paths, it is most important that the polarization of the antennas at both ends of the path use the same polarization. In a linearly polarized system (horizontal or vertical), a misalignment of polarization of 45 degrees will degrade the signal up to 3 dB and if misaligned 90 degrees the attenuation can be 20 dB or more. Likewise, in a circular polarized system (right-hand or left-hand), both antennas must have the same sense. If not, an additional loss of 20 dB or more will be incurred.

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