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Old 28-Sep-2013, 2:05 AM   #1
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Join Date: Sep 2013
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new help to identify UHF Yagi


I recently found this old antenna lying in my attic unused. Correct me if I'm wrong, but I believe it to be a directional UHF Yagi. I would guess it to be around 20-25 years old. The boom measures 70.5" It has 14 directors, 1 reflector, and I'm assuming the longer one on back is the driven element?

I'm trying to figure out who might have made it, what kind of gain it is capable of, and it's frequency range. I'm assuming it could probably achieve 15-16dbi? Wishful thinking?

I'm currently using this as my OTA antenna. I removed the old 300 ohm wire pair, and screwed on a 300 to 75 ohm balun. It's currently installed in an attic (temporary), and it does okay considering it's horrible placement. I have plans to mount this on an old Directv J-pole with a mast extender to get it outside and above the roofline.
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Old 30-Sep-2013, 10:07 PM   #2
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Location: Greater Seattle Area
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Given the age of the antenna, it's possible that it's engineered for the older UHF band (CH-14 to CH-83). Still, it looks like it's in great condition. I don't know the make or model number, but a 14 director Yagi should give you the gain you've estimated.

Here is the output from "VK5DJ's YAGI CALCULATOR"


Yagi design frequency =600.00 MHz
Wavelength =500 mm
Parasitic elements contacting a square section metal boom 25.00 mm across.
Folded dipole mounted same as directors and reflector
Director/reflector diam =6 mm
Radiator diam =6 mm

The abbreviation "IT" means "Insert To", it is the construction distance from the element tip to the edge of the boom for through boom mounting

262 mm long at boom position = 30 mm  (IT = 118.5 mm)

Single dipole 232 mm tip to tip at boom posn =130 mm 	(IT = 103.5 mm)
Folded dipole 257 mm tip to tip at boom posn =130 mm 	(IT = 116.0 mm)

Dir	Length	Spaced	Boom position	IT	Gain	Gain
(no.)	(mm)	(mm)	(mm)		(mm)	(dBd)	(dBi)
1	230	37	167		102.5	5.4	7.6
2	227	90	257		101.0	6.9	9.1
3	225	107	365		100.0	8.1	10.2
4	222	125	490		98.5	9.1	11.2
5	220	140	630		97.5	9.9	12.1
6	218	150	779		96.5	10.7	12.8
7	216	157	937		95.5	11.3	13.4
8	214	165	1102		94.5	11.8	14.0
9	213	172	1274		94.0	12.3	14.5
10	211	180	1454		93.0	12.8	14.9
11	210	187	1641		92.5	13.2	15.3
12	209	192	1834		92.0	13.6	15.7
13	207	195	2029		91.0	13.9	16.1
14	206	197	2226		90.5	14.2	16.4

Spacings measured centre to centre from previous element
Tolerance for element lengths is +/- 1 mm

Boom position is the mounting point for each element as measured
from the rear of the boom and includes the 30 mm overhang.The total boom length is 2256 mm including two overhangs

The beam's estimated 3dB beamwidth is 31 deg

A half wave 4:1 balun uses 0.75 velocity factor RG-6 (foam PE)  and is 187 mm long plus leads

Here are some construction details for a folded dipole

Measurements are taken from the inside of bends
Folded dipole length measured tip to tip = 257mm
Total rod length =543mm
Centre of rod=272mm
Distance HI=GF=106mm
Distance HA=GE=133mm
Distance HB=GD=161mm
Distance HC=GC=272mm
Gap at HG=10mm
Bend diameter BI=DF=35mm

If the dipole is considered as a flat plane (see ARRL Antenna Handbook) then its resonant frequency is less than the flat plane algorithm's range of 10:1
If the well is dry and you don't see rain on the horizon, you'll need to dig the hole deeper. (If the antenna can't get the job done, an amp won't fix it.)

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Old 1-Oct-2013, 4:44 AM   #3
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Posts: 12
Thank you for the reply and information.

I also had the assumption that this antenna was used on the older UHF band. I say this because I believe it was used in conjunction with a VHF antenna, which I also dug out of my attic. There is also evidence left behind that they split the VHF and UHF signal into one with a splitter that I found.

If it was in fact designed to pick up 14-83, then it makes sense that it picks up all my local channels with the antenna pointed at roughly 300*

Couple of other interesting tidbits....

Even though it appears that this was strictly designed for UHF, it picks up KFME (VHF-Hi) with ease.

Also, I was pleasantly surprised to see that it picks up KVRR very well off the back end of the antenna.

I believe this thing has some great potential for my purposes. Even though the attic placement is horrible, it still manages to pick up everything in green on the analysis report. I am having periodic issues with KXJB-DT & KVLY-DT pixelating and sometimes dropping momentarily (SNR will drop below 16), but I blame the placement for that. I can't wait to see what this thing will do on a 5ft mast mounted on my old dish j-pole!
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