Building Center-Fed Dipole for 39-45MHz Antenna

[217]

I thought the radiation pattern of a dipole is perpendicular to its length, shouldn't dipoles be vertically mounted, how come yours is horizontal?

I use it for DX.

 

[majoco]

Here's my scanner/receiver antennas - the folded dipole is cut for 125MHz aero band - the orange box contains the normal 300/75 ohm TV un-un which goes to RG6 cable - the dipole is cut for 75MHz and the pill box contains the 75 to 50 ohm balun to RG58 cable. The discone feeds an Icom R7000. The little pill box at the bottom of the discone is just an 'umbrella' for the connector, no self-amalgamating tape or anything - stays perfectly dry!

 

[prcguy]

TV type 75 to 300 ohm transformers are only rated down to 54MHz. They are not so good at 54MHz and will have further degradation at the CHP freqs of 39 to 45Mhz.

A regular old 1/2 wave center fed dipole is ok and using a 1:1 choke balun at the feedpoint is a good idea. I can supply easy cheap plans for a choke balun if needed. You would have to get the vertical dipole well away from a mast or it becomes a two element directional Yagi. It will probably not cover the entire 39 to 45MHz range perfectly but it will work ok.

Why not shop for a used surplus military antenna that covers the VHF lo range? They are inherently broad band and will cover at least 30 to 76MHz with no tuning and newer models will cover 30 to about 90Mhz. You can find OE-254 bicones or Cobham COM210Bs on Ebay for good prices and you would never be able to design and build an antenna that can compete with those.

I take that back. I have a design (a partially poached design) to make a broad band "bowtie" antenna from about 30 to 90MHz using some wire and PVC pipe, plus an easy to make custom 4:1 balun. I'll post some plans in the future after I make one and document it. You would also be able to transmit and receive on this antenna from CB to about 90Mhz with up to 100 watts.

 

[StaticDischarge]

I understand. The SWR meter is of no use if you are not transmitting. Antenna analyzers are <$500. However, That's still as much or more than most people spend on a scanner.

The Arrow ground planes are relatively cheap and will be designed to your specs and tested. Telewave does offer discount to hams. We just put up a two bay vertical dipole array at one of our repeaters. Beautiful antennas.

Antenna analyzers, are not all over $500.00..
Try this one on for size: (FA-VA5 600MHz Vector Antenna Analyzer Kit) Mine works really good! And it was less than $200.00 shipped! Although you will have to wait a bit to get it due to demand but it's worth the wait!

FA-VA5 Antenna Analyzer Kit

www.sdr-kits.net

 

[majoco]

The lowest frequency TV channel in NZ was 45MHz - you have to assume that the antenna manufacturers would use balun cores that would accommodate this frequency which I why I used one for 75MHz.

TV type 75 to 300 ohm transformers are only rated down to 54MHz.

 

[prcguy]

The OP is in the US and the few US type TV transformers I've measured were surprisingly lossy.

The lowest frequency TV channel in NZ was 45MHz - you have to assume that the antenna manufacturers would use balun cores that would accommodate this frequency which I why I used one for 75MHz.

 

[RFI-EMI-GUY]

The OP is in the US and the few US type TV transformers I've measured were surprisingly lossy.

The OP can always try it out and see if it works. Then make his own, or buy a wide band 4:1 or 6:1 balun part from Minicircuits for cheap. I bought the minicircuits balun to make a standard dipole for some testing and got a bunch very cheap.

He could have even made his antenna by now (and proven us all wrong) in the time this thread has run.

 

[RFI-EMI-GUY]

I use it for DX.

Are you finding the DX signals are refracting to horizontal polarization in a reliable manner? Curious because I have a diversity reception idea in mind.

 

[217]

Are you finding the DX signals are refracting to horizontal polarization in a reliable manner? Curious because I have a diversity reception idea in mind.

It is often said that single-hop Sporadic E signals tend to remain in the original transmitted polarisation. I have not noticed any difference Rxing 33/46 Mhz horizontally after ten years of verticals.

 

[kandrey89]

I ordered another type of 300 to 75 ohm transformer from amazon and opened both of them, they are both using ferrite cores, no capacitors (I don't know what that trash is about).

So the reason I was having such a terrible quality before is because I was using a 3/8" pipe with 1 side cut for 40MHz and another for like 150MHz, and this simply didn't work.

This time I took 10AWG stranded wire with insulation, and cut it to 42MHz length, both sides, attached it to 300-75 TV transformer, and voila, clear reception on a half wave dipole. Much less static noise than the existing CHP feed on radioreference. Right now it's on my room floor, but I plan on putting it in the attic either vertically or horizontally, probably the former, put my raspberry pi and scanner up there too, and stream the feed.

For now I've been streaming the feed using my phone's browser by connecting to icecast (running darkice) on the raspberry pi.