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I recently acquired a Harmon Kardon TU 915 AM/FM tuner from a member of this forum. On the Tuner Information website there is a review of this unit that says that using a 280kHz ceramic GDT IF filter for the first filter stage in this unit using a Bill Ammons gain stage improves the signal quieting ability of this tuner. Has anyone measured the amount of improvement?
Also, the station I want to receive is on 101.1mHz and there is a strong station on 100.9mHz. The station I want to receive is broadcasting a HD subcarrier. Would it be better to change to a 180kHz GDT filter or even a 150kHz GDT filter? If either of these filter bandwidths was installed would it be necessary to have the tuner re-aligned?
Follow Ups:
I copied some discussions on the HD self-noise and circuits for adding extra filtering for the same.
With the change to colder weather and the holidays season, I may not get much done, especially on outside antenna changes.
One antenna modification I thought about is the Körner 15.11 FM only antenna design approach mentioned on the Beezley web site. I could add extra reflectors to the back end of the existing antenna and increase the front to back ratio, thus knocking down the offending station. I had not thought of the idea of stacked antennas with one pointed at the offending station and with reversed phasing - not a bad idea! Thanks for the suggestion.
Using a rhombic antenna in the attic is not viable due to the construction of our house. The house has aluminum foil backed roof decking for better heat rejection. Unfortunately it also creates a Faraday shield against any RF signal. Even our outside vertical walls have similar aluminum shielding built in. We can only get a cell phone signal at windows! If I did a rhombic antenna it would have to be entirely outside. That might still be a possibility. Such an antenna would not be very visible.
Today is an overcast day and the signal from WRR is producing full quieting. It is surprising how much difference weather conditons make at 60+ miles from the desired station.
If they are not within say 30 degrees of each other a directional antenna would give you a LOT of help. And wouldn't hurt under that arc.
Besides making the wanted station sound better, and any other desired stations you might point it at.
We have all my desired stations (and heaps of commercial ones) all on the one big high telecom's tower on top of a big hill. Plus two sets of repeaters for all the RF holes and valleys. Lots of hills.
So, I don't need a rotator.
The repeaters help with our in-car reception of FM.
If you've read this, a response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
WRR FM 101.1 is a 98kW station located in Dallas, TX at 184 degrees from true North at a distance of 61.7 miles with a signal level of -50.3dBm.
The offending station is KWRD 100.7 also a 98kW station at 42 degrees from true North at a distance of 4.7 miles with a signal level of -10.6dBm.
I use an outside Winegard FM only antenna with 13 elements oriented to the WRR location in Dallas. This data was obtained from FM Fool's website based on physical address. You can see that the offending station is quite stout. Without an outside directional antenna picking up WRR is a really difficult task for any tuner. About the only other thing I could do on the RF levels would be to put a balun transformer right behind the tuner, convert to 300 ohm twin lead and put a tunable trap there to try to create a suck-out at 100.7 mHz. I don't currently have the needed 9-180pFd variable capacitor to make the trap. If the trap creates too broad a suck-out it could reduce the signal level of WRR making acceptable noise free reception of the desired station worse than it is currently. It might be worth a try though.
I still think that I need a slight boost in the IF gain and selectivity of my TU-915 by changing at least one of the current IF filters using one of Bill Ammons boards that provides moderate gain plus selectivity, or maybe even an extra stage of filter & gain compared to the existing 3-filter arrangement.
A great FM tuner is expensive- just look at high-end Kenwood, McIntosh, Sansui, Accuphase tuners. Being an inexpensive mod, I would simply install the IF filter kit.
and connected out-of-phase with the main antenna, you'd need a combiner on the mast as well. If you are not using a rotator, there's just the issue of vertical spacing on your existing mast, 6ft being the minimum.
Of course the offender would have to be a station you'll never bother listening to! ;-)
Better IF filters -might- do the job on their own, and the tuner has a wide/narrow switch?
Option 2: ONLY IF you have a truly large loft space with a truly long diagonal in the direction of your WRR!? Getting an even bigger signal from it than any retail Yagi or LPD could manage - even stacked. A rhombic wire made from twin lead, with sides well above (double?) the wavelength at 100Mhz of about 10 feet. The existing Wineguard could then be used for an even deeper notch, or for any other station you've wondered about. When the 3 element would be needed for the notch.
If it were me I'd get the filters, then consider how much quieter and clearer I'd like WRR to be, as well. You'd also know how problematic the offender is after the filters.
I've posted the URL for the rhombic article below.
Good luck.
If you've read this, a response is preferred.
Warmest
Timothy Bailey
The Skyptical Mensurer and Audio Scrounger
And gladly would he learn and gladly teach - Chaucer. ;-)!
'Still not saluting.'
Oops! I was wrong on the direction of KWRD 100.7mHz, the angle from true North is 346 degrees, which is essentially off the back side of my Winegard FM only antenna. A recheck using FM Fool shows the signal level at my location of this station is -9dBm, a very stout signal.
Actually the TU-915 does have three ceramic IF filters from the factory. There are 2 transistors in a common emmiter configuration between the 1st and 2nd filter and between the 2nd and 3rd filter. Those seem to be there for impedance matching plus enough gain to offset the filter losses. If I do the change(s), I plan to order parts pre-selected for correct IF center frequency from Bill Ammons, which might eliminate the need for realignment (I hope).
This unit replaces a Dynaco FM-5 made in 1976 which had two ceramic IF filters in it. It did a fair job dealing with the adjacent channel strong station, but fell a little short on quieting of the desired station. The Harmon-Kardon TU-915 is considerably better on quieting of the desired signal and does even better at rejecting the adjacent channel station.
Thanks for the thoughts.
You'll need to go with 2 matched 150's at 10.70 MHz to get the job done. I have a similar situation here. Bill can do this matching and sorting.
Also try tuning to the offending station and fine tune the rotation of the antenna for the weakest signal. Then tune to the desired station. In your case this may help find the best compromise, but maybe not. This technique works best when the two stations are at 90 degrees, and you fine the offender to be in the sharp side null. In your case, you are relying on the antenna F/B ratio, tuner capture ratio, and IF filtering.
Bob
Ok, it appears you are amost there with thew TU915.
The ceramic filters vary in center frequency say a few percent. Thus, you would have to send your existing filters to Bill to be exactly matched up. Make sure the matched to original replacement filters go back in the same spot.
As filters become more narrow, the audio is affected slightly. 180kHz is as narrow as I would venture. Cascaded filters actually makes the filter skirts more narrow at each downstream IF section.
Once a IF filter is changed alignment is needed. Also, same manufature same part number ceramic filters have different center frequencies- production variance. Have a pro tighten up the IF filter chain & align it.
I believe a stock TU915 has only two ceramic filters, not so good for high selectivity. A post detection filter may also be necessary due to the HD subcarrier dispite adding narrow IF filters such as 180kHz plus an additional filter. (google search hd radio self noise for more info).
There are other tuners that provide out off the box higher performance. In other words you may end up with more money in the TU915 vs another tuner that costs more up front. But, then again the TU915 may be an audio high performer that another tuner may fall short.
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