In praise of the VHF yagi antenna.

[Stereoholic]

Good evening all, this afternoon I took down my DIY FM yagi for a bit of maintenance and while I was at it I took down some dimensions.

The reflector element is about 61.75 inches long and it is spaced about 24.25 inches away from the driven element, behind it. Regards, Felix (vk4fuq) aka catman.

Thanks for posting that, as I was just about to ask.

 

[chicks]

Hey, Felix, what do you think about a Rhombic antenna for FM? Looks super cheap and easy (assuming I can find some cheap twin-lead, not as easy to do as it once was). Would be perfect for the garage ceiling; disadvantage is that it's fixed, while a Yagi would be easy to rotate to catch the stations in the "wrong" direction, like KDVS and KVMR. However, I wonder if it would have enough gain to pick up Bay Area stations, which are behind a low (1,000-2,000 ft.) mountain range from me, but only some 50 miles distant as the crow flies?

http://docs.google.com/viewer?a=v&q...5p4zGT&sig=AHIEtbT1l1n95TBX_wHS4pD0Hk31qBX0og

Edit: Just found this great resource, which shows the reception possibilities for all FM stations at a given location:

http://www.fmfool.com/index.php?option=com_wrapper&Itemid=29

Very interesting, I was assuming that most of the FM stations were using the huge TV towers in the SW quadrant. Turns out the only decent Jazz station in town (KXJZ) is in the NW quadrant, and is considerably weaker than the Classical station (KXPR) at my location. Now I know where the antenna will need to be pointed. Poor reception of KDVS and KVMR appears to be not just their orientation, but their severely limited power. If I want to get a Bay Area station, looks like I should first try KQED, with 110KW of power!

 

[steerpike2]

You can also stack Yagis, one on top of the other on the same pole, if you need more gain. And if that's not enough, side by side, so ending up with 4 (or more of you have space!).

 

[catman]

G'day all, for reception of FM stations, a simple yagi is preferred for several reasons. A Rhombic 'could' be used but they tend to be quite large and bulky, even on the VHF bands, whereas a yagi tends to be more compact in size.

Also a yagi is more suited to horizontal or vertical mounting depending on the required signal polarisation, rhombics are less easily configured in this respect.

Yes Yagi's can be 'stacked' for additional gain, however optimised 'stacking' a is complex procedure requiring precise techniques involving optimised stacking distances between the individual yagi antennas and complex 'phasing lines' feeding each antenna in a 'stacked' array.

A simple 'single' yagi of good performance is much easier to get going, in my opinion. Regards, Felix (vk4fuq) aka catman.

 

[steerpike2]

Stacked Yagis were quite common here in the 1980s, where different television shows were broadcast by very narrow beam to different ethnic suburbs. If you wanted to watch shows that weren't intended for your 'colour', you needed an array of 4 or 6 Yagis.

 

[fortycoats]

Stacked Yagis were quite common here in the 1980s, where different television shows were broadcast by very narrow beam to different ethnic suburbs. If you wanted to watch shows that weren't intended for your 'colour', you needed an array of 4 or 6 Yagis.

That's very interesting. Never figured on them trying to prevent certain people from watching certain material.

 

[BinaryMike]

That's very interesting. Never figured on them trying to prevent certain people from watching certain material.

We're certainly drifting off topic here, but I understand that radio receivers were deliberately manufactured with reduced sensitivity in Nazi Germany -- to keep the population from listening to non-German broadcasts. I'll shut up now...

 

[steerpike2]

That's very interesting. Never figured on them trying to prevent certain people from watching certain material.

Yes, off topic, but I'll explain briefly - because it is interesting:
Certain actors trade unions, and TV companies didn't want their tv programs shown to "White South Africa" as part of trade embargoes, so the SABC was allowed to broadcast them only if they were restricted to largely black audiences. That was not too difficult to arrange because the physical location of residential areas determined the ethnic composition.
But as with so many things supposedly embargoed, there was always a way around it. Either buy a bigger antenna; alternatively, all of the embargoed shows could be hired on VHS from places which has contacts in the broadcast areas with recorders.

 

[ng0k]

I was at a BBQ yesterday and the host showed me some old antenna stuff he had laying around. I think his dad was a ham. Anyway he had a 2 meter - 10-element yagi. Typical gain around 15db in freespace. An FM version of that would pull in the fringe stations!

 

[catman]

G'day mate, the basic problem with designing a yagi for the FM broadcast band is trying to obtain uniform gain right across the fiull FM band as it is very wide in terms of frequency (88 to 108 MHz).

That is why many commercially made FM antennas are 'peaked' for certain parts of the band. A simple three element yagi has the advantage of good gain (around 7 dbd) over a wide frequency range. Regards, Felix (vk4fuq) aka catman.

 

[jheu02]

If you want homebrew antennas, here's the best site I can point you to:

http://ham-radio.com/k6sti/

I built a version of Peter Korner's 17.5 but later went back to my modified RS6 since I could mount it higher. I didn't have a rotator bearing and didn't want to ruin my rotator or chimney. But here's a couple pics from the construction and in place.

http://groups.yahoo.com/group/fm_antennas/photos/album/1275940038/pic/list

John

 

[Hepcat]

There are VHF yagi's all over my neighborhood. No need to build one if I didn't already have a big VHF/UHF with rotator etc. I easily pick up FM stations 100 miles away. Most folks would give them to you if you took it down.

 

[ManFromPorlock]

Years ago I had Radio Shack's biggest VHF-TV Yagi (forget how many elements but it was the one that cost around $70 in the 80s) mounted on about a twenty foot mast and pointed roughly southwest from Orono, Maine. I used it to feed both TV and FM receivers and could get stereo and video from Portland (about 120 miles) with no problem. In fact, fairly often Boston and Providence, Rhode Island, TV stations would come in, and Providence is 240 or so miles away. Anomalous propagation, of course, but the antenna had enough gain to detect it when it happened.

 

[jheu02]

There are VHF yagi's all over my neighborhood. No need to build one if I didn't already have a big VHF/UHF with rotator etc. I easily pick up FM stations 100 miles away. Most folks would give them to you if you took it down.

Yeah, but it's a fun project. After all, this is a hobby!

John

 

[catman]

G'day all, as a active ham radio operator with a passion for antennas and antenna design, I always make my own antennas for what ever frequency is needed.

Buying a ready made antenna would always be an absolute last resort! Regards, Felix (vk4fuq) aka catman.

 

[chicks]

I was moving gear around in the mancave yesterday (too much gear!), and pushed the rabbit ears all the way down to get them out of the way. Just turned on the receiver, and now the only Jazz FM station in town, which had always shown up full strength, but suffered lots of multipath noise/distortion, is suddenly noise free! Stereo light is full on, can't tell if it's full strength since the receiver's meter is suddenly on the blink, but damn, it sounds great!

 

[catman]

G'day mate, you might have stumbled onto a bit of a 'secret' here, as for the best performance a 'rabbits ears' antenna should be approximately a 'half wavelength' long at the intended frequency, neglecting any feedline SWR mismatch.

The formula is (approximately) length overall in feet: 468/ frequency. For example 468/98 (middle of the band, more or less) = 57.306 inches, or half that figure per side. Regards, Felix (vk4fuq) aka catman.

 

[jheu02]

I was moving gear around in the mancave yesterday (too much gear!), and pushed the rabbit ears all the way down to get them out of the way. Just turned on the receiver, and now the only Jazz FM station in town, which had always shown up full strength, but suffered lots of multipath noise/distortion, is suddenly noise free! Stereo light is full on, can't tell if it's full strength since the receiver's meter is suddenly on the blink, but damn, it sounds great!

For more info on rabbit ears, click here:

http://ham-radio.com/k6sti/rabbit.htm

This is one of the many antenna alternatives that Brian Beezley modelled. The site I earlier posted is the main index.

John