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Old 26th April 2011, 01:33 AM   #21
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Quote:
Originally Posted by DF96 View Post
Two wideband dipoles suitable phased would do the trick.
How are you proposing to achieve a suitable phase relationship across a 4:1 range of frequencies?

2C39A and 3CX100A have gain to >1GHz. 3GHz I think.

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Old 26th April 2011, 01:37 AM   #22
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Quote:
but do you really need an amp?
I'm thinking two amps!

Obviously two coat hangers is not a fully optimized antenna system, but it performs far better than expected. Many $50 to $100 antennas on the market aren't either. Many of them are amplified, again with a less than optimum amplifier. So I will experiment with amps just for a learning experience. I have been building ham radio amps for years, and I design two way radio equipment at work. So, the next step is amplification. Down the road, I'm looking at a ultra cheap rhombic antenna. I built one about 15 years ago for the 902 MHz ham band and it worked quite well for TV even picking up VHF channel 3 (63 MHz) from Tampa occasionally, about 200 miles away.

Why two amps?

The spectrum analyzer reveals several signals just above the noise floor. I have maybe 10 db of cable loss at the high frequencies, which adds directly to the system noise figure. I know my old spectrum analyzer has a noise figure of at least 10 db probably worse. I don't know what the noise figure of a modern TV is. Putting an amplifier at the antenna with a noise figure of 1 db and sufficient gain will do wonders to lower the system noise figure. This amp needs a low noise figure, good gain, and good intermod performance. I may put the rhombic on a rotor and use it for ham radio and general snooping, so frequency response out to 1.3 GHz would be nice. I am looking at the SPF-5122Z from RFMD (under $5 at Digikey) and the HMC617LP3 from Hittite Microwave (hard to get).

I have been using a Radio Shack amplified splitter to drive long cables (over 100 feet) to TV's throughout the house, and a pair of ethernet TV tuners. The analyzer revealed that the current amp cuts off several channels above 650 MHz and has severe gain tilt, so it must go. I am currently driving 4 seperate cables from its outputs, and some of them have multiple resistive splitters at the far ends. The amp here needs outstanding intermod performance, good power output capability, reasonable noise figure, and frequency response to 1.3 GHz. The Avago MGA-30689 is the winner here. I have already been pounding these with LTE signals from 700 MHz to 2.4 GHz, at +20 dbm peak power. LTE demands linearity, and more linearity, and flat frequency response across a 20 MHz channel.

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Thanks George!....now the girls can watch all their soaps in the break room
And you will be found responsible for the loss of productivity. The plant where I work has a large international population. The computer network got overloaded during the World Cup because everybody was watching the games on their desk computers. They finally put the games on in the break rooms, the gym, and the lobby.
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Old 26th April 2011, 02:40 AM   #23
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I believe TV receivers are required to have a NF of 14dB for sale in the US. Certainly many manufacturers no longer strive to achieve performances in line with those sought in the past, signal conditions being generally better than they were and many receivers being fed from satellite receivers or other set-top boxed or players.

A MMIC with a NF of 1dB is certainly excellent, but one of the widely available devices with 3dB NF is probably perfectly adequate at the masthead. Minicircuits have the PHA-11+ with very good intermod and 75 ohm match, but this is a recent introduction, and I can't find anywhere to buy it yet.

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Old 26th April 2011, 02:49 AM   #24
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Execelent work I used to love building my own antenna's when I lived in naples.
I never ever bought a commercial one when I was playing around in the 11 meter band and I would blow away most antron 99 setups with a 3/4 wave ground plane.

Thanks for this interesting read. jer
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Old 26th April 2011, 12:37 PM   #25
DF96 is offline DF96  England
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Quote:
Originally Posted by wakibaki
How are you proposing to achieve a suitable phase relationship across a 4:1 range of frequencies?
Two equal lengths of cable? Impedance matching is the difficult part. How about a two-input preamp, with signal combining at the output?
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Old 27th April 2011, 02:37 AM   #26
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Two equal lengths of cable?
The problem is the high angle radiation, which is most of the radiation for a horizontal dipole. You can stack cophased horizontal dipoles vertically and improve the vertical firing angle and the high angle lobes, but the performance is spacing and height above ground dependent and consequently variable with frequency. I think with fixed spacing you'd want to be able to alter the phase relationships between the dipoles, and keep the spacing under a wavelength even at the highest frequency of interest. You could rack them for spacing, but it's not a simple arrangement then. A sufficiently broadband dipole will be a biconical which offers constraints on spacing anyway.


Radio Antenna Engineering - Design of a Horizontal Half-wave-dipole Antenna System

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Old 27th April 2011, 03:32 AM   #27
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Unknown to me the guy across the street went to Best Buy last weekend to buy a TV antenna. He got a silver plastic box about 6 inches by 8 inches and 1 inch thick, made by Philips. It cost about $40 and according to the instructions has "18 db of gain". It plugs into the wall so it has an amp. The salesman told him that it would work OK but the box clearly says UHF only.

He mounted it on a pole at about the same height as my antenna. His antenna does receive the two VHF channels and most of the Miami based UHF channels. It only gets some of the West Palm Beach channels. Time for some wire hangers....

I started the layout for the mast head amp. The board will be capable of using the RFMD SPF-5122Z or the Avago MGA-633P8. Both are stocked at Digikey. Tiny little SMD creatures though.
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Old 27th April 2011, 11:35 AM   #28
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Quote:
Originally Posted by wakibaki
The problem is the high angle radiation, which is most of the radiation for a horizontal dipole.
Only when it is near the ground. A UHF TV antenna is likely to be sufficiently high (in terms of wavelengths) to be virtually in free space. Also, the ground will be lossy so a poor reflector.

I accept that vertical spacing of biconical antennas may be a problem. There is an interesting variant: a stacked/phased DIY bowtie TV antenna (see Google). This uses 4 bowties.
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Old 27th April 2011, 01:50 PM   #29
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Commercial bowties have been around since the 1950's, probably older but that's how far back my memory reaches.

Looking back into my memory. I remember making channel 20 Yagis from broom sticks and coat hangers in the late 60's to pick up the blacked out Dolphin games from Fort Myers. That died when Tampa got an NFL franchise and WINK started covering Tampa instead of Miami. The plans and formulas were in the old ARRL handbooks.

Amplifiers were nonesixtent then since there were few devices available to the hobbiest that worked at 500 MHz. I tried acorn tubes but only made oscillators!
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Old 28th April 2011, 12:55 AM   #30
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A relaxed attitude is certainly an asset when confronting any design problem, but a too-casual approach often results in a mediocre performance.

In the past, when moving house, I have myself taken a wire coathanger, pulled it into a loop, straightened out the hook to a rightangle bend and forced it into the centre of the co-ax receptacle on the TV, where it was self supporting. I got a picture, but I didn't spend a lot of time congratulating myself on my design expertise, more my good luck.

I can take an opamp and a FET out of my junk here and with a few additional resistors and caps produce a device that will amplify audio. I wouldn't be in a hurry to claim hi-fi performance though. Antenna design is amongst the most demanding of electronics disciplines, and I doubt that the contributors in this thread would be as enthusiastic about a thrown together tube amplifier.

This said, I really am trying to take a more upbeat view of things, so I shan't be offering any more criticism. I'm glad it worked out in this case, and if you decide to post your 3.0 I'll happily make such constructive suggestions as might occur to me.

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